Direct filtration of Biesbosch water and Algae and water treatment in the Netherlands: 3rd Direct Filtration Seminar

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Di

rect

filtration of Biesbosch water

and

Aigae and water treatment in the Netherlands

3rd_D_irect_Filtra_tion _Sem_inar

May 1993 B.Petrusevski, A. Vlaski. A.N.van Breemen and G.J.Alaerts

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Delft

Rapp CT )elft University ofTechnology WMG-Gez.

Faculty of Civil Engineering

Gro up ofSanitary Engineering&Water Management Env iro nm ent al andSan itaryEngineeringSection

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3

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_{Direct Filtration Seminar}

DIRECT FILTRATION OF BIESBOSCH WATER

AND

ALGAE AND WATER TREATMENT IN THE NETHERLANDS

B.Petrusevski, A.Vlaski, A.N. van Breemen and G.J.Alaerts

In cooperation with:

*

Internationallnstitute for Infrastructural Hydraulic and Environmental

May 18, 1993

Engineering (mE) - Delft

Technische Universit

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D

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Faculteit CiTO

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Bibliotheek Civi

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Stevinweg 1

2628 eN

Delft

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'11-0~ TU Delft

Delft University of Technology

Faculty of Civil Engineering

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CONTENTS

Introduetion . . . i State of the Art: Direct Filtration of Biesbosch Water . . . 1.1 Effect of Ozone and Potassium Permanganate on Direct Filtration Performance . . . . 2.1 Algae and Water Treatment in the Netherlands, Problem Analysis and Treatment

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Introduction

Within the studies "Direct filtration of Biesbosch water" and "The relevanee and removal of cyanobacteria in water treatment", meetings are organized with experts from waterworks and related institutions on a regular basis, to present recent research findings.

At this meeting, third in series, three papers will be presented as follows: "State of the Art: Direct Filtration of Biesbosch Water"

B.Petrusevski M.Sc.

"Effect of Ozone and Potassium Permanganate on Direct Filtration Performance" B.Petrusevski M.Sc.

"Algae and Water Treatment in the Netherlands, Problem Analysis and Treatment Strategies in Five Water Companies"

A.Vlaski M.Sc.

Technische Universiteit Delft Faculteit CiTG

Bibliotheek Civiele Techniek Ste vinweg 1

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch WaterW

State of the Art:

Direct Filtration of Biesbosch Water

B.Petrusevski, A.N.van Breemen, and G.J.Alaerts

1.0 Introduction

This presentation summarises basic information on direct filtration, and demonstrates the main research findings, related to the performance of simple in-line direct filtration.

The results reported are part of a comprehensive ongoing research programm "Direct filtration of Biesbosch water" undertaken jointly by Delft University of Technology and International Institute for Infrastructural, Hydraulic and Environmental Engineering (IHE Delft). The main research goal of the study is to assess the applicability of direct filtration to water from Biesbosch reservoirs. The key issue addressed in this study is coagulation of algae and other particles and their subsequent removal in rapid sand filters.

2.0 Background on direct filtration 2.1. General lnformatlon

Direct filtration is a water treatment process without separate floc separation step, either sedimentation or flotation, which traditionally precedes filtration. The conventional and direct filtration flow schemes, for treatment of surface water, are shown in Fig.l. The addition of coagulant in conventional surface water treatment plants is followed by flocculation unit and floc separation units. However, direct filtration treatment process does not include a floc separation unit before filtration. In addition, a flocculation unit is sometimes not part of a treatment scheme. Direct filtration treatment, without a separate flocculation unit, is known as "in-line" or "contact filtration" . Direct filtration supported by a flocculation unit is known as "floc-filtration".

The advantages of direct filtration emerge from process simplicity. This treatment process is economically very attractive. Ifappropriately applied it can substantially reduce investment and operational costs. The reductions of capital costs up to 50 %and chemical costs up to 30 %have been reported. Produced amount of sludge is significantly reduced due to lower dosages of chemicals. In addition, produced sludge is contained in a single stream, filter backwash water, thus simplifying a sludge treatment installation. Finally, direct filtration requires less space for a plant construction.

The disadvantages of direct filtration are: shorter filter runs in comparison with filters operating in conventional treatment schemes and consequently higher backwash water consumption. Direct filtration treatment process provides shorter water residence time and only one partiele removal barrier. As a consequence, skilled and qualified personal is required for operation of treatment plant. Ifraw water contains arsenic, the application of

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3rd Direct Filtration Seminar

Conventlona. treatrnent

"DirectFiltration of Biesbosch

Water-AaVII Co.gul.nt floooul.tlonf - - - l '1008 • •par.tlon r.pld . a n d filtration AaVII Co.gul.nt I I I L.. -&- - I Direct filtratlon rapld . a n d flItratIo"

Fig.l. Conventional and direct filtration flow scheme for surface water treatment

direct filtration will increase the arsenic concentration in sludge. In the recent direct filtration experiments, we found four times higher arsenic concentration in the sludge in comparison with the sludge coming from the conventional water treatment plant (WTP Kralingen). Direct filtration process can be applied only to raw waters of appropriate quality, meaning that turbidity, colour, suspended matter and algal concentration should be low. However, the acceptable upper limits for these raw water quality parameters differ depending on a souree consulted (table No.I). The AWWA Filtration Committee's report on direct filtration, which compiles data from existing direct filtration plants and completed pilot plant studies, elucidates that colour exceeding 30-40 Hazen units and turbidity greater than 15 FTU on a continuous basis, could give problems. Raw waters with less than 40 units of colour, turbidity consistently below 5 FTU, iron and manganese concentrations of less than 0.3 mgll and 0.05 mgll respectively and algae count not exceeding 2000 ASU, appear to be, according to this report, a perfect candidate for direct filtration.

Table No.1

PARAMETER UNIT ACCORDING TO:

Edzwaldi1987 AWAi'980 Culpi1977

turbidity FTU < 20-30 < 5 < 25 < 200 low

a and r-and -and

colour Pt-Co < 30-40 < 40 < 25 low < 100

algal count ASU/ml low < 2000 < 1000

iron mg/l

-

< 0.3

.

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-3rd Direct Filtration Seminar "Direct Filtration of Biesbosch

Water-The useful diagram to define a preferred treatment option is given on Fig. 2 (Janssens, 1992). The selection of a floc separation process is based on suspended solids expressed as turbidity and algal concentrations expressed as chlorophyll-a. According to this diagram, direct filtration is a preferred treatment option for raw water sourees with turbidity below 10 FTU and chlorophyll-a below 10 J,tg/l.

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Fig.2. The diagram for selection of an appropriate floc separation process (Janssens et al., 1992)

2.2. Applications of Direct Filtration

Direct filtration is attractive treatment alternative for construction of new municipal water treatment plants if a available raw water souree is of appropriate quality. In addition, existing conventional water treatment plants, purifying surface water of high quality, may be converted into the direct filtration treatment mode, by-passing a sedimentation or a flotation unit and in some cases also a flocculation unit. Alteration of treatment mode will reduce significantly operational costs, and amount of produced sludge.

Simple in-line direct filtration treatment may also be very attractive for industrial applications, particulary for production of water of somewhat lower quality.

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch Water"

In a view of direct filtration ability to produce water with very low partiele concentration, this treatment process may be considered as an appropriate pretreatment technique for membrane filtration.

Applications of direct filtration in the field of waste water treatment are also known. There is a large number of full scale water purification plants that operate in a direct filtration treatment mode. In this report four of them will be mentioned for particular reasons:

a. The Los Angeles Aqueduct Filtration Plant, as the largest direct filtration treatment plant. The treatment scheme of this plant and basic the process parameters are indicated in Fig.3. b. The Treatment Plant Prieure, Geneva, treating water from lake Leman, as a direct filtration plant using raw water with similar phytoplankton concentration and dominant algal species as Biesbosch water. The treatment scheme of this plant and the basic process parameters are given in FigA.

c. The direct Filtration Treatment Plants of the Wahnbach Reservoir Association, as an example of a two stage direct filtration plant.

d. The treatment Plant Kraayenberg, as the only application of direct filtration in the Netherlands. The plant supplies water to local industry. The produced water is of considerably lower quality in comparison with drinking water standards.

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Fig.3. The Los Angeles direct filtration plant - the treatment scheme and basic process

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3rd Direct Filtration Seminar "Direct Filtration of Biesboscb

Water-Fig.4. The water treatment plant in Prieure, Geneva - the treatment scheme and basic process parameters.

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3.0 Direct Filtratien of Biesbosch Water 3.1. Initial Research Considerations

"Direct Filtration of Biesbosch Water"

To fulfil the research programm objectives, reduce as much as possible the duration and complexity of pilot plant experiments, and at the same time establish the operational conditions that reflect adequately Biesbosch water treatment, the introductory research activities were focused on:

a. Sealing-down direct filtration

A bench scale and a pilot plant set-up for direct filtration modelling (the modified jar test apparatus - MJTA, and the bench scale direct filtration pilot plant - BSDFPP) were developed (design, supply of equipment and materials, implementation, and trial operation). It was found that these experimental set-ups are providing reliable and reproducible indication of direct filtration performance, as well as good correlation with previously developed a larger scale unit (the automated mobile direct filtration pilot plant - MDFPP).

b, Selection of raw water to be used in the experiments

As a preliminary step we challenged a routine approach in the studies dealing with algal flocculation, which is based on the use of model waters with pure algal cultures. Itwas found that the growth media have a pronounced influence on algal coagulation behaviour and their removal efficiency. Consequently, the use of model water and the comrnon extrapolation of the results in earlier model studies applying algal cultures, 10natura! conditions seems to be highly questionable. Conceivably the better approach is to use natura! algae from water body intended to be used as the raw water source. However, that approach is faced with experimental difficulties caused by the low algal concentration in natura! waters over long periods of the year. This obstacle was resolved by concentration of natura! algae with the tangential flow membrane filtration system. Results obtained reflect that high concentration factors, of up to 20, may be applied without affecting algal viability and cell recovery rates. Accordingly, it was decided that natura! Biesbosch water, pre-concentrated during period of low algal concentration, should be used in the experiments.

c. Water quality parameters to be followed in the experiments

To assess the performance of direct filtration treatment process the following water quality parameters were selected to be followed:

- turbidity,

- partiele concentration and partiele size distribution, - algal concentration,

- DOC and UV at 254 nm,

- residual iron and manganese concentration, - zooplankton concentration,

Further on, we also plan to checked some additional water quality parameters, (e.g.the assimilabie organic carbon - AOC, bromate etc.), for the optimised direct filtration treatment line.

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3.2. Target Filtrate Quality

Very high requirements for filtrate quality were established: - Turbidity =:; 0.1 FTU;

- Residual iron =:; 0.1 mg/l;

"Direct Filtration of Biesbosch WaterW

- Chlorophyll-a =:; 0.1 J.'g/l (Interim standard of The German Association of Drinking Water Reservoirs);

- Very high (preferably complete) elimination of particles

>

6 J.'m as predietors of Giardia cysts and Cryptosporidium;

The requirement for very high partiele removal efficiency was established to achieve high algae and pathogenie microorganisms elimination rates. The Surface Water Treatment Rule (US Environmental Proteetion Agency states: "... systems are required to meet design and operating criteria to ensure removal or inactivation of at least 99.9 percent of Giardia cysts..."

3.3.Initial Experiments

Initial experiments were designed to assess the performance and ability of a simpie, in-line, direct filtration treatment, to cope with seasonal water quality fluctuations. In these experiments the MDFPP was used. The following operational conditions were applied:

- simple in-line operational mode; - no pre-oxidation;

- no separate flocculation unit; - no pH correction ;

- coagulant: ferric-sulphate; dose 0.5-1.5 mg/l; - filtration rate 7.5-15 m/h;

- dual (anthracite/sand) and multi (anthracite/sand/garnet) filter media;

The results obtained with the MDFPP under the previously defined conditions cao be summarise as follows:

- low filtrate turbidity: 0.10-0.12 FTU with three layer filter media and 0.11-0.14 FTU with dual filter media (Fig.6);

- particles, mainly algae, are passing treatment;

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch

Water-- algal removal efficiency is different for different algal species; elimination rates from 5 to 100% have been found (Fig.5);

- three layer filter media is consistently producing filtrate of superior quality

(turbidity, residual iron and partiele count) comparing to dual media filter (Fig.6,

7 and 8); use of three layer filter media appears to be an essential prerequisite to cope with rising standards for drinking water quality (e.g. expected reduction of residual iron concentration to 0.05 mg/l);

- filter runs are acceptably long (40-50 hours); - filter ripening period is long (several hours);

- coagulation pH has strong impact on filtrate turbidity, partiele concentration and iron residuals (Fig. 9, 10and 11),in contrast to a common assumption that the iron salts used as coagulants are effective over a wide pH range; ,it was found that coagulation at higher pH values (> 7.9) results in superior filtrate quality, which is not in agreement with the traditional considerations that coagulation at lower pH values enhances treatment efficiency.

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bloDl. di•• 100 110 10

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Fig. 5. Removal of different algal species obtained in the MDFPP during summer periode Coagulant: ferric sulphate, 1.0 mg Fe(lII)/I, F1-dual media filter, F-2 three layer filter, filtr. rate 10 m/h. Algal species; R.min. - Rhodomonas minuta, m.al. - single cells of Walgae, Cyl.dia. - cylindrical diatoms.

Itwas also recognised that turbidity, which is commonly used as an evidence of a successful treatment performance is not a good indicator ofpartieleconcentration at low turbidity levels (Fig. 12). Appropriately, it may be concluded that, in addition to turbidity, partielecount is an essential parameter required to assess the performance of direct filtration.

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-3rd Direct Filtration Seminar

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch Water-JOO , . - - - , s J 2 I 200 L - _ . . . . L . - _..._ ..._ - - ' - _ - - ' L - - - - ' o ç Run time (lJ)

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Inadequate algae removal efficiency is the problem which clearly emerges from these initial experiments, design to study simple in-line direct filtration process. Accordingly, considerable research efforts were done to study agglomeration and removal of algae. It was found that, in addition to variation of algal behaviour among different species, the algal growth stage has very strong impact on their agglomeration properties and removal efficiency. Summarising the available information in literature it was identified that the following algal properties facilitate their penetration through treatment:

- algal size (1-10 J.'m),

- surface characteristics (negative surfaee charge, eell wall composition); - shape of the algae (e.g.long needles ofStephanodiscus hasuzschiîï, - extra cellular organic matter (BOM) produced by algae.

Itwas also recognised that the conventional analytical techniques (turbidity, chlorophyll-a, particles and algae enumeration) have limitations in their ability to advance our knowledge and understanding of phytoplankton behavioral response to specific treatment. Itappears very appropriate to apply a video system (a video camera attached to an inverted microscope) for in situ phytoplankton observations. The system was used in combination with the tangential flow filtration set-up. Video systems have not been used before in the phytoplankton treatability studies. In the experiments performed, a video technology has demonstrated the capability to overcome the shortcomings of conventional techniques and provide more insight into the phytoplankton behaviourial response. For example, in the experiments with Biesbosch reservoir water, it could be demonstrated that the algal motility is of decisive importance for agglomeration and removal of numerous algal species (e.g. Rhodomonas,

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Cryptomonas, Pandorina, Navicula, Chlamidomonas ï , This finding is rather significant and

in contrast to common concepts which do not emphasise algal motility.

3.4. Conclusions

The results obtained during this initial experimental work lead to the foUowing conclusions: Simple and smalI scale experimental set-ups were developed for direct filtration modelling, which results correlated weU with the larger scale pilot plant - the MDFPP. This set-up is considered to reflect adequately real-life situation;

Natural Biesbosch water, pre-concentrated during period of low algal concentration, should be used in the experiments;

Turbidity is not an adequate parameter for high water quality standards; partiele count is necessary to provide supplementary information;

Three layer filter media is superior in comparison with dual media;

The coagulation pH value, if not foUowed by pH correction before filtration, has strong influence on filtrate quality; coagulation at pH

>

7.9 is required to minimise filtrate turbidity and residual iron concentration;

Simple in-line direct filtration process, without appropriate pretreatment (e.g.oxidation), cannot achieve required particles and algal removal efficiency.

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Water-Effect of Ozone and Potassium Pennanganate on Direct Filtration Performance

B.Petrosevski, A.N.van Breemen, and G.J.Alaerts

1.Introduction

The previous presentation clearly indicated that the simple in-line direct filtration treatment process, without appropriate pretreatment, applied for purification of Biesbosch reservoir water, would be faced with some of the plankton related problems. The most serious problem is poor removal of particles, more specifically penetration of algae and other particles in the algal size range, through treatment. The additional problem, rapid clogging of a filter and surface or cake filtration, caused by high concentration of filamentous and large algae and zooplankton, may also be experienced. Furthermore, the extra cellular organic matter (BOM), produced by algae, under certain conditions may hinder the processes of coagulation and flocculation. .

A possible remedy for these problems may be raw water pretreatment. Appropriate pretreatment, preceding direct filtration, may reduce or completely eliminate these problems. Micro-strainers, applied as an initial step may efficiently eliminate large and filamentous algae and zooplankton. A separate flocculation unit, in addition to other possible benefits, will prolong filter runs by reducing head loss development in a filter bed. Pretreatment techniques such as application of miero-strainers and a separate flocculation unit were already discussed on the previous meeting. This presentation will focus the problem of poor removal of algae and other particles in direct filtration. Preliminary results from the experiments designed to study the effect of pre-oxidation with ozone and potassium permanganate on direct filtration efficiency will be presented.

It is known that oxidants can interact with impurities in such a way that more efficient particles removal can be realized by subsequent treatment. Significant improvements in algal removal efficiencies and direct filtration performance have been reported following chlorine or ozone pretreatment of raw waters.

Accordingly, the initiating experiments, were performed to scan the effect of pre-treatment with ozone, hydrogen peroxide and potassium permanganate on partiele and algae removal efficiency and direct filtration performance. The other oxidants such as chlorine and chlorine dioxide are discarded in a view of their heal hazardous by-products. The results from these initiating experiments were presented on the previous meeting, held in April 1992. One of the closing conclusion from that meeting states: " .. ozone and potassium permanganate are showing the promising oxidative and coagulating effects and are an interesting option for a future investigation ... ". This conclusion was the starting point for further experiments aimed at improving algae and partiele removal efficiency in direct filtration treatment. Consequently, numerous experiments with ozone and potassium permanganate were performed to study in detail the effect of these oxidants on direct filtration performance.

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2.0zone

2.1 Theoretical background

Ozone is one of the strongest oxidants. It has been increasingly used as a pre-oxidant, as an

altemative to chlorine, because ozone does not produce the chlorinated by-products.Itis very

powerful for taste, odour and colour removal. Itcan act as a coagulant aid. However, the

mechanisms in which ozone acts as a coagulant aid are not yet known precisely. Itis known

that preozonation causes a shift in partic1e size distribution towards larger partic1es and formation of colloidal partic1es from dissolved organic matter. This improves organic

removal during subsequent treatment. Itis also known that ozone easily reacts and splits the

algal EOM dissolved in the water or adsorbed on other partic1es. It has been reported that

preozonation reduced a coagulant dosage and extended the length of a filter run. In addition to the genera! enhancement of the coagulation process, there are c1ear evidences that preozonation significantly improves algal agglomeration and removal.

However, investigations conducted to study preozonation, dernonstrated a number of related problems:

- with certain waters preozonation failed to show a beneficial effect;

- the use of ozone does not necessarily have a favourable impact on the agglomeration

of all algal species (e.g. filamentous algae);

- the considerabie increase of the assimilable organic carbon - AOC in the oxidised

water;

- preozonation can increase meta! residuals in filtrate by stabilizing coagulant microflocs; the problem is more pronounced at low coagulant to total organic carbon

(TOC) ratios, at high ozone levels (above 0.3 mgOimg TOC) and when a coagulant is an iron salt;

- ozonation of waters containing bromide (e.g. rivers Meuse and Rhine), may lead

to the formation of bromate at levels suspected to be hazardous to hea1th.

Though the observations from full scale plants applying ozone, as weIl as research findings, are not consistent, Reckhow et al. (1986) outlined that there is a genera! consensus regarding the following points:

(a) low ozone dosages

«

3 mg/I; most often 0.5-1.5 mg/l) are most effective; higher

dosages can lead to deterioration of the coagulation process;

(b) it is important to add a coagulant at the point of ozonation or shortly afterwards, and

(c) a certain critica! concentration of organic matter must be present in raw water to observe

the coagulation effect of ozone.

It may be summarised that the pretreatment with ozone can improve the performance of

direct filtration in terms of effluent turbidity, filter run length and algal removal efficiency. However, in a view of possible side-effects, as weIl as relatively high investment and operational costs, the final conclusion regarding ozone application should be drawn only after

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detailed study, preferably including pilot plant experiments

2.2 Preozonation and direct Flltratlon of Biesbosch Water 2.2.1 Experimental set-ups

A semi-batch and a continuous flow ozone reactors were developed to study the interaction between ozone and Biesbosch reservoir water.

The semi-batch ozone rector was used in the experiments with the modified jar test apparatus (MITA). Two glass jars of 5.5 and 8.51 were used as reaction vessels. The reactor achieves ozone transfer efficiency of 60% . The concentration of ozone in produced ozone gas was 23 mg/I. As a preliminary step a reliable procedure was established, to allow very accurate predictions of ozone dosages transferred to raw water.

The continuous flow ozone reactor was developed to study the effect of preozonation on the performance of pilot direct filtration plants, namely the bench scale and the mobile direct filtration pilot plants ( BSDFPP and MDFPP respectively).The reactor consists of avertical, transparent, PVC column, 133.5 mm in diameter and 4.5 m high. The reactor is designed for counter current flow, and has maximal capacity of 1200 lIh. The reactor achieves very high ozone transfer efficiencies. The gas leaving the ozone reactor was found to be praeticall y ozone free.

2.2.2. Results

2.2.2.1. Optimisation of ozone and coagulant dosage

Data from existing fuIl scale plant that successfuIly apply preozonation, as weIl as completed pilot plant studies, show that the beneficial coagulating effect of ozone is pronounced at low ozone dosages, below 3 mg/l. The ozone dosages within the range 0.5-1.5 mg/l have most often been reported as optimal. In addition, there are reports indicating that preozonation reduces the required coagulant dosage.

Accordingly, the first set of experiments was designed to check the validity of these findings for preozonation applied to Biesbosch water. The experiments were conducted with the MJTA. In these experiments, Biesbosch reservoir water five times preconcentrated was used to provide algal concentration that correspond to an average spring levels ( the experiments were conducted during period of low algal concentration). The graph presented in Fig.l. demonstrates the effect of ozone and coagulant dosage on partiele removal efficiency. The experiments were repeated with different coagulant dosages (0.5, 1.0 and 1,5 mg Fe(III)/l). The results shown in Fig. 1 suggest the foIlowing conclusions:

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch WaterW

- there is an optimal ozone dosage, in this particular case 1.8 mg Oil;

- an increase of coagulant dose improves partiele removal efficiency; the highest

tested coagulant dosage of 1.5 mg Fe(III)Il, gives the highest partiele removal

efficiency. /00 ,0 - _

--

--p'" -I ~

----J:)

.90 , , ' "<. Fo/l.:> I " •••• I , ' ...

, . '

...

~ "0 ~._---'t:. Fc/l.a '. .. .- - - -

,

., •••••t:.

,,

70

,

I

/

/ +

so / Fc/O.S SO + 40 JO I . . . - - - ' - _ - ' -_ _- - - ' - _..._..L...---""_....J a / 2 J

Ozont' dOSIlIt' (mI/I)

Fig .1. Effect of ozone and coagulant dosage on partiele removal efficiency.

It is important to note that very efficient partiele removal (96.5%) was achieved with the

MJTA, This set up has shallow and clean, sand filter bed, thus suggesting that the removal efficiency in real, ripened, filter will be even higher. The optimal ozone dosage found in

these experiments (1.8 mg 0₃ consumedll) is comparable with values reported in the

literature. Finally, the experiments with the MJTA, did not show that preozonation could reduce required coagulant dosage.

2.2.2.2. Correlation between optimal ozone dosage and algal concentration

The algal population in Biesbosch water, and in any other surface water reservoir is showing seasonal fluctuations. The level of chlorophyll-a, which is commonly used to indicate the concentration of algae, fluctuates in Biesbosch water in a wide range. The concentrations

bellow 1}lg/l are common during winter periods. On the other side levels as high as50}lg/l,

have been recorded during spring algal blooms. The yearly average chlorophyll-a level is

about 5 }lg/l.

The next set of experiments was designed to assess the effect of algal concentration on the required ozone dosage. The concentration of algae in raw Biesbosch water was increased with the tangential flow filtration system, to different previously defined concentration levels.

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Water-The experiments were conducted with the semi-batch ozone reactor and the MJTA. Preozonated model water (pre-concentrated raw Biesbosch water) was exposed to the standard modified jar test procedure. The results from these experiments are presented in a condensed form in Fig.2. The graph clearly shows that an increase in partiele or algal concentration, elevates an optimal ozone dosage. A very strong correlation between the optimal ozone dosage and a partiele count was found (the correlation coefficient was found to be 0.92). Accordingly, during the period of a low concentration of algae (November - March) , significantly lower ozone dosages can be applied, thus reducing the operational costs and a risk linked with the formation of the ozonation by-products (e.g. bromate).

2.00 , . . . . - - - , ... llIQ .~ o.so

""

~ o /000 2000 JOOO

PartkIt' conc. (par'/ml)

Fig.2. Effect of partiele concentration on optimal ozone dosage.

2.2.2.3. Point of coagulant addition

Summarising data from literature and successful full scale applications of ozone as a coagulant aid, Reckow et al. (1986) stated that it is important to add a coagulant at the point of ozonization or shortly afterwards. The next set of experiments was design to assess the importance of point of coagulant addition (or the time between ozonation and coagulation) for the performance of the preozonation-direct filtration treatment applied to Biesbosch water. These experiments were conducted with the semi-batch ozone reactor and the MJTA. The results (Fig.3) clearly show that a delay in coagulant addition, after preozonation, have strong negative effect on filtrate quality. Itwas found that shorter time between preozonation and coagulant addition produced filtrate with the lower partiele count and turbidity. The partiele counts in the filtrate were found to be lowest for time interval between preozonation and coagulation of about one minute. This findings were very important as a design criteria used to establish the continuous flow pilot plant ozone reactor.

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch Water" 1000 , - - - , ~

~

~ ₁₀₀ 1llI.

-s

....

§

_çoo C:) ~ ~ .u

::

...

400 ~ ~

....

~ 200

....

~ ~ o 8 12 IÇ 20 Time (Min)

Fig.3.Effect of time between ozonation and coagulant addition on partiele concentration in

the MJTA filtrate.

2.2.2.4. Preozonation in combination with polymers

The microscopie examinations of the MJTA effluent show that preozonation enhances

agglomeration of algae and other particles. However, formed flocs are too small to be very efficiently removed by filtration. Accordingly, the next set of experiments was designed to assess the capability of polymers applied as coagulant aids to enhance the agglomeration of already formed microflocs, and accordingly to improve removal efficiency in a filtration step. The preliminary MJTA experiments were conducted with four different types of polymers:

- Wisprofloc P (cationic) and Wisprofloc N (nonionic), both starch derivates and - Superfloc C573 (cationic) and Superfloc AlOO (anionic) both synthetic products. The applied experimental procedure included preozonation at an optimal ozone dosage, coagulation with ferric sulphate (1.5 mg Fe(III)/l), addition of a polymer (0.15 mg/I) and filtration at a constant filtration rate through small scale filters. All polymers tested have shown a strong beneficial effect on turbidity and partiele removal efficiency. However, the cationic polymer Superfloc C573 produced the best results. Therefore, this synthetic cationic polyrner was selected for further experiments

The next set of experiments was designed to study the effect of the polymer dosage on a

process performance. The experiments were performed with the MJTA, under the same

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_

..

,~

-3rd Direct Filtration Seminar "Direct Filtration of Biesbosch Water-between 0 and 1.0 mg/l were tested. The results given in Fig. 4 and 5 show clearly that the application of polymer improves filtrate quality. An increase in polymer dosage, reduces partiele and iron concentrations in filtrate. Application of cationic polymer at somewhat higher dosages, in direct filtration treatment process supported,by preozonation and coagulation with iron coagulant, resulted in practically complete elimination of particles that

cao

be detected by the HIAC particle.counter (the size range 2.7-150 JLm). The other benefits, reduction of filtrate turbidity and residual iron concentrations were also observed.

Itshould be pointed out that these results were obtained with a very simple experimental set-up the MJTA, that features only short and clean bed, thus suggesting that even higher removal efficiency caobe obtained in fuU scale applications.

O.SO , . . - - - , JOO zoo /00 b I .... O.JO ~

~

..

\

It'+ + E -"" 0 •.10 \ \ +....•...+ ~

:

~ I "" C)~

::::

\ l&.; 0./0 \ \ Go_ ~--e ----~... 'n 0.00 0 0.00 0 •.10 0.40 o.~o 0.10 /.00 - - - PC ..+.. Turb.

Fig.4. Effect of polymer dosageon the MJTA filtrate turbidity and partieleconcentration.

O.SO r---....,

0.00 ' - - - _ - ' - - _ - ' - - _ . . . . L - _ . . . . o . . . - _ . . . . J

0.00 0•.10 0.40 0.#0 0.10 /.00

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2.2.2.5. Pilot plant experiments

The additional experiments with the MDFPP and the BSDFPP were performed to verify the results obtained with the MJTA. The BSDFPP provided a possibility.to establish and assess in parallel four different direct filtration treatment trains. The treatment alternatives examined are shown in Fig.6. The common coagulant, ferric sulphate, dosage was 1.5 mg Fe(lII)/I; the dosage of polymer, Superfloc C573, was 0.6 mg/l and pretreatment with ozone ( 1.8 mg consumed Dil) was introduced in the treatment lines F-1 and F-2. The filters used in these experiments were equipped with three-Iayer beds (anthracite/sand/garnet). Coagulated Biesbosch water was filtered at the constant filtration rate of 10 m/h was applied. The graphs showing the impact of treatment composition on direct filtration performance are given in Fig. 7and 8. Raw Bleaboac:tt

-Fe Polymer

F-4

Fe

F-3

Fa Ozone Fe Ozone

F-1

F-2

Fig.6 . Treatment alternatives examined with the BSDFPP.

The results obtained with the BSDFPP cao be summarised as follows:

(a) preozonation has a pronounced beneficial effect on partic1e removal efficiency is ; direct filtration treatment train inc1uding preozonation and application of polymer achieved 99.7%

partic1e removal efficiency, more than one log unit higher in comparison with the treatment trains without preozonation (Fig.7);

(b) direct filtration treatment that inc1ude pretreatment with ozone (F-l and F-2) easily produced filtrate of required turbidity (below 0.1 FfU), (Fig.8);

(c) the residual iron concentrations in filtrate from the preozonated treatment train without polymer (F-l), were very low (

<

0.06 mg/l); application of the polymer reduce further iron residuals.

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch WaterW /000 , . . . . - - - , G-_~_--~---~---~----O Fe 03+Fe 03+Fc+C573 /0 ... /00 + + + + + + Fc+C573 ~ ~ ~ ~

t:

a

I 2 JI...-~-...-.o...---'---'---'---J a

Filter rUD time (11)

Fig. 7. Effect of treatment mode on partic1e count in the BSDFPP filtrate.

0.50 , . . . . - - - ,

~

0040 4II I I

~

I I ,_, ~ ,_• I

b

I\ 0.30 ,

...

, I ~ Q •I ~ \ •

.

~ '\ \ I I

""'

I 0.20 +. ~

.

'. \\b..---e-_

....

~ '. \ ----E) Fe .1

....

• ~

."

Po.CS73 tl.; 0.10 6."..._'" _..+_.._{.. ..} ..+ _03.Po " "~----6 OJ ••••CS7J 0.00 0 2 3 4 5

F.iJtt:r rUL] t.imt: (b)

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch WaterW Microscopie examinations revealed that the filtrate from treatment train with preozonation, without application of polymer (F-l), did not contain any algae except Walgae

«

2.5 JLm).

The direct filtration treatment train that in addition include the use of the polymer as a coagulant aid (F-2) produced filtrate practically free of algae and other particles larger than

1JLm.

The supplementary filter runs were performed with the MDFPP. This set-up is considered to reflects adequately full scale applications. Raw Biesbosch water was preozonated (1.8 mg 0₃ consumed/l) and coagulated with 1.5 mg Fe(III)/l. The results obtained with the MDFPP (Fig. 9 ) correlated weIl with the results found with the smaller scale units, namely the MJTA and the BSDFPP, thus clearly confirming the beneficial impact of preozonation on direct filtration performance.

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2.2.2.6. Conclusions

The results from the experiments conducted with different experimental set-ups, designed to study the role of preozonation for direct filtration of Biesbosch water, lead to the following conc1usions:

*

Preoxidation with ozone distinctly improves the ability of direct filtration to remove partic1es and algae present in the raw Biesbosch water;

*

Direct filtration process that inc1udes preozonation and the application of the cationic polymer as a coagulant aid, provides practica1ly complete partiele and algae elimination;

*

Coagulating effect of ozone is dosage related; there is a c1ear correlation between the optimal ozone dosage and algal concentration;

*

Filtration through the three layer filter media, that follows preozonation and coagulation with an iron coagulant, produces effluent with the very low iron concentrations.

In conclusion, the obtained experimental results demonstrate that preozonation significantly improves overall direct filtration performance. Direct filtration treatment supported by preozonation, can easily produce filtrate of the targeted quality: turbidity

<

0.1 FfU,

chlorophyll-a

<

0.1 ",gil, exceptionally low partiele concentration, and Fe

<

0.1 mgll, thus completely fulfilling establish research program objectives.

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3. Potassium pennanganate 3.1. General characteristics

Water-Potassium permanganate (KMn04) is another powerful oxidant, which is nevertheless not

frequently applied as a pretreatment in direct filtration treatment schemes. Traditionally,

KMn04is used in water treatment for the removal of dissolved iron and manganese, and taste

and odour control. Application of potassium permanganate as an algicide for algal growth control in the reservoirs has been studied and reported by several researchers in the late 1960's. Bemhardt and Lusse recently reported that the inactivation of zooplankton by potassium permanganate resulted in significantly increased zooplankton removal efficiency in a flocculation filtration system. A few recent studies indicated that potassium permanganate, applied as a pre-oxidant, may behave as a coagulant aid. Jun Ma and Guibai Li (1992) found that the pretreatment with potassium permanganate resulted in an enhanced coagulation-flocculation process, reducing the coagulant dosages and improving turbidity removal. Petrusevski et al. (1992) reported that preoxidation with potassium permanganate clearly enhanced algal removal efficiency. The effect of potassium permanganate as a coagulant aid may be related to the insoluble manganese dioxide hydrates (for convenience

further on notified as MnO_{z) ,} formed in the pH range used in water treatment. Mn~

enhances the flocculation kinetics and acts as adsorbent as a consequence of its large surface

area (300 mZ/g).

Drawbacks, related to the use of permanganate, frequently precluding its use in water

treatment, are the introduced violet colour and the elevated residual manganese levels. 50 far

there is no evidence that oxidation with potassium permanganate produces by-products hazardous to hea1th.

3.2. Pre-oxidation with potassium pennanganate and direct filtration of Biesbosch water 3.2.1. Overview of preliminary experiments

Preliminary experiments were designed and performed to study the effect of preoxidation with potassium permanganate on partiele removal efficiency and overall direct filtration performance. Applied oxidant dosages ranged from 0.5 to 5 mg KMnOil. The results from these experiments were partly reported on our previous meeting. The main findings may be summarised as follows:

- Pre-oxidation with potassium permanganate at all dosages tested distinctly improved particulate and algal removal efficiency. Pre-oxidation typically halved partiele concentration in the filtrate. These findings were confirmed by algal enumeration as weIl as by the HIAC partiele counter (Fig. 10 and 11);

- The filtrate turbidity was higher for pre-oxidised samples (treatment scheme F-2); the difference was more pronounced at the beginning of the filter run, and diminished

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mainly in the size range from 0.45 to 2.7 JLm. Pre-flocculation (treatment scheme F-1), showed beneficial impact on filtrate turbidity, reducing elevated levels caused by Mn02 (Fig. 12);

- An increase of residual manganese concentration as a consequence of pre-oxidation was noticed. A strong positive correlation with the applied potassium permanganate dosage was clearly observed (Fig. 13). Potassium permanganate dosages of 1 and 2 mgll elevated manganese level in the filtrate significantly above the maximum admissible concentration of 50 JLgll (according to the European drinking water directives). Oxidation with 0.5 mg KMnOil, produced filtrate with residual manganese below the maximum admissible concentration for drinking water. Nevertheless, manganese content was still considerabie, and unacceptably high from the operational viewpoint. Additional analysis revealed that almost all the manganese present in the filtrate is retained on the0.45JLm membrane suggesting that manganese was present in the form ofMn~precipitates.

- a light violet colour in the filtrate was observed 'only at high potassium permanganate dosages (2 mgll or more);

JOO - F-J - F-2 ----. F-J

,---"

"

..../

...

-/ / / / , / 7

s

J 2 J J 00 L - _ . l . - _ . . L . . - _ . . L . . - _ . . L . . - _ . . . . L . . - _ . . . . L - - - - l

o

Run time (h)

Fig. 1O.Effect of preoxidation with KMn04on partiele concentration in the BSDFPP filtrate. F-l and F-2 represents treatment trains that include KMn04

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Water-_ F - J

ilBI

F-2

~F-J

PRrtJcJtJ siztJ

Fig.Ll , Effect of preoxidation with KMn04 on filtrate partiele count and partiele size distribution. F-I and F-2 represents treatment trains with KMn04.

/.00 . . . - - - . - F-/ - F-2 ._--. F-J 0.20 7 J 2 / 0.00 l - - - I _ - - - L _ - - - L . _ - - L - _ . . . . L . . - _ . . L . . . - - - J o Run time (11)

Fig. 12. Effect of KMn04 and pre-flocculation on filtrate turbidity. F-l and F;.2 represents treatment trains that include KMn04; F-I in addition includes pre-flocculation.

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3rd Direct Filtration Seminar "Direct Filtratien of Biesbosch Water-D.JO , - - - , 0.00 O.S J.D s» KMD04 dostl (mI/I)

•

Fig.13. Manganese concentration in the BSDFPP filtrate as a function of applied KMn0₄

dosage.

2.2.2. Goals of the further experiments with KMn04

Having in mind the results from the preliminary investigations, the subsequent experiments were aimed at:

a. maintaining or preferably improving partiele and algae removal efficiency, and

ultimately reaching targeted water quality (chlorophyll-a

<

0.1 p.g/l or 99% algal

removal efficiency),

b. eliminating the potassium permanganate drawbacks (elevated filtrate turbidity and residual manganese concentration).

An increase of manganese concentration in the filtrate was of particular concern. In Europe the maximum manganese concentration in drinking water must be below the level defined by

the EEC directive of 50 JLg/l. However, in a view of possible problems in distribution

network, even at such low manganese concentrations, an interim goal was established to

eliminate practically cornpletely residual manganese from filtrate.

3.2.3. Conventional Treatment Alternatives

To reach established goals, impact of a number of treatment process parameters, on the residual manganese concentration, partiele removal efficiency and overall direct filtration performance, was initially assessed. The importance of the following process parameters was studied:

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- contact time of potassium permanganate;

- application of a separate flocculation unit with different flocculation conditions (residence time and velocity gradients);

- application of sodium sulphate as an reducing agent; - coagulant, ferric sulphate, dosages and

- application of dual and three layer filter media.

Prolonged potassium permanganate contact time enhanced the precipitation of permanganate into the form of manganese dioxide, thus reducing the concentration of dissolved manganese. However, the extended contact time itself couldn't eliminate the potassium permanganate related drawbacks.

The next set of experiments was designed to evaluate the possible beneficial impact of a separate flocculation under different conditions on direct filtration performance, and specifically the permanganate related problems. The results from numerous modified jar test experiments have shown that flocculation improved partiele and turbidity removal and reduced residual manganese levels. The flocculation G*t values within the range of 40000 to 70000 were found to be optimal. However, the residual manganese concentration in the filtrate was still to above 50 J,Lg/l. The effect of flocculation, at different G*t values, on residual manganese and filtrate turbidity is given in Fig. 14 and 15.

3SIJ ~---, o + + v .. A A + cr.o. ~ AA • o + v

•

A v

•

v I J L . . . - - - - ' - - - " - - - ' IJ I DIJ 3DD JDD (T~ollU.tJI} FJoccuJ.tioD Glit

• 081 • OJ, 0 OS, • ou • a'., • ou,

Fig.14. Effect of flocculation on residual manganese in the MITA filtrate.

It has been reported that the application of sodium sulphite as a reducing agent accelerated the precipitation rate of potassium permanganate and reduced the residual manganese concentration in the full scale direct filtration plant effluent. To verify the validity of this finding for direct filtration supported by preoxidation applied to Biesbosch water, we performed a set of experiments, under the similar conditions as reported in the literature. The

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3rd Direct Filtration Seminar 1.0 . - - - , P 0.8 0 v v 0.0

o

100 200 JOO (TJzous.nds) FJoccuJstioD allt

Water-Fig. 15. Effect of flocculation on turbidity of the MJTA filtrate.

experiments performed have shown that the application of the reducing agent slightly reduced manganese concentration in the filtrate. However, the remaining residual manganese was still considerably above 50 p.g/l. The application of sodium sulphite did not show any effect of filtrate turbidity and partiele concentration.

The next set of experiments was designed to evaluate the ability of coagulation at higher coagulant, ferric sulphate, dosages to reduce the problems related to the use of potassium permanganate and to produce filtrate of targeted quality. The results from these experiments show that an increase in coagulant dosage reduces filtrate turbidity, partiele count and residual manganese. Nevertheless, the coagulant dosages tested ( up to

<

2 mg Fe(III)/1 considered to be the highest tolerabie dosage for direct filtration ) couldn 't reduce sufficiently residual manganese and produce filtrate of acceptable quality.

Our earlier experiments, with simple in-line direct filtration without any pre-treatment, have shown strong beneficial impact of the three layer, anthracite-sand-garnet filter media on filtrate turbidity and the residual iron concentrations. Therefore, the additional set of experiments was designed and conducted with the BSDFPP to compare the effect of the dual, anthracite-sand, and the three layer, anthracite-sand-garnet, filter media on filtrate quality, specifically the residual manganese concentrations. These experiments have clearly demonstrated reduction of the residual manganese concentrations in the filtrate of three layer media (Fig. 16). The manganese levels were below the maximum admissible concentration defined by EEC directive (50 p.g/l). Nevertheless, the residual manganese concentrations were considerable and above the intemally established limits. However, having in mind the beneficial impact of the three layer filter media on residual manganese, iron, filtrate turbidity and partiele concentration it was decided to use this type of filter media in further experiments with the BSDFPP and the MDFPP.

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60 r - - - ,

... til _:JO ::J ~ ~ ~ /0 0 0 I :J J 4 S # FjJt~r rU/1 tim» (b)

Fig.16. Effect of dual and three layer media on residual manganese in the BSDFPP filtrate.

To summarize, the examined process alternatives, namely extended permanganate contact time, a separate flocculation unit, higher coagulant dosages, the application of sodium sulphite and use of the three layer filter media, although improved filtrate quality were not capable to eliminate completely the potassium permanganate related drawbacks and produce effluent of targeted quality.

3.2.4. Treatment alternative favouring charge neutralisation mechanism

The examined treatment alternatives failed to resolve the problems related to the application

of potassium permanganate, the most serious problem being increased manganese

concentrations and turbidity of the filtrate. These problems are caused by the manganese

dioxide hydrates that easily pass through treatment. To understand this phenomenon comprehensively and provide appropriate solution it was necessary to collect available information about the potassium permanganate chemistry.

The simplified potassium permanganate oxidation reaction in natura! water may be presented as follows:

KMn04

+

H20

+

[organics, Fe2+, Mn2+ ..

l -

Oxidation by-product

+

Mn02~

In this reaction the manganese in the potassium permanganate ion is reduced from the +7 valenee state to +4 in the reaction product, manganese dioxide hydrate. This compound on the one hand aids the treatment process, but on the other has properties that facilitate its

stability and penetration through treatment. When initially precipitated _MnOzparticles have

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- the size of0.3- 0.4 JLm;

- negatively charge at pH levels above 3; - large surface area 300 m2_/g;

- a very strong adsorbing capacity;

- a polymerie structure (actually occurs as a hydrous oxide).

The negative surface charge the size range were considered responsible for Mn~ stability and poor removal efficiency. Appropriately we established the following hypotheses as the starting point for further experimental work:

a. The manganese dioxide hydrate partic1es are passing treatment and causing the problems of increased residual manganese and turbidity levels;

b. The agglomeration of these very small partic1es is an essential prerequisite to remove them efficiently in a filtration step;

c. Reduction or neutralisation of Mn02 negative charge is necessary to enhance their agglomeration rate.

To verify these hypothesis and resolve the potassium permanganate problems, additional series of experiments were designed with the coagulation conditions that favour the charge neutralisation mechanism, Literally, three sets of experiments were conducted:

- the experiments designed to investigate the effect of coagulation with iron coagulant at different coagulation pH values, with an emphasis on a lower pH range;

- the experiments designed to study the application of a cationic polymer as a sole coagulant;

- the experiments designed to assess the efficiency of treatment that inc1ude use of the iron salt, ferric sulphate, in combination with the cationic polymer, Superfloc C573.

3.2.4.1. Effect of coagulation pH

The ability of iron salts, which are used in water treatment as coagulants, to destabilize the colloidal partic1es can be explained by their aqueous chemistry. When this salts are added to water they will form the aquacomplexes Fe(H20)l+. These complexes are stabIe only in a very acid media. When added to natura! waters, they will pass through series of hydrolytic reactions in which H20 molecules are replaced by OH-ions :

In conventional water treatment, the quantities of iron used as a coagulant are much higher than dictated by the solubility limits of their hydroxides. Accordingly, Fe(H20)63+

.transforms to a metal hydroxide precipitate, passing through previously given reactions. However, these reactions, as weIl as the maximum solubility limits of iron hydroxides are pH dependent. Coagulation at lower pH favours the formation of positively charged iron hydroxides species, thus enhances the destabilization of colloids by charge neutralisation. As

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Water-previously indicated, the Mn02 particles formed as an end product of the permanganate

reaction are in the colloidal size range and have the negative surface charge. Accordingly, it was assumed that the coagulation with an iron salt at the pH values that favour charge neutralisation would enhance removal of the Mn02particles, and reduce related drawbacks.

With these theoretical background in mind the experiment with the MJTA were conducted to assess the effect of coagulation pH on efficiency of preoxidation-direct filtration treatment, and specifically removal of residual manganese from filtrate. The process conditions applied in these experiments were: pre-oxidation with 0.7 mg/l KMn04 , coagulation at variabie pH

with ferric sulphate, dosage 1.0 mg Fe(III)/l, followed by pH correction and flocculation at constant pH=8.0 and finally filtration through smal1scale filters at the constant filtration rate of 1.8*lOo3m/s.

Results shown in Fig. 17 and 18 confirmed that the coagulation pH value has pronounced effect on residual manganese and partiele concentration. The graphs show distinct reduction of manganese concentration and partiele count with pH reduction. However, reduction of the coagulation pH value down to 6, a level which is already considered to be too low for practical application, couldn't itself reduce residual manganese to acceptable limits. However, the results partIy confirmed the hypothesis previously established.

1.0 r - - - , 800 200 600 400 ~-""t,d,1.I

,,

,

,,

,

.,.

,

,,

,

,,,

I .Q..

,,

",71-,

»<; " " ...+'

,

"

,

....-e- Turb. --+- PC 0.6 004 0.8 0.2 8 7 0.0 L . . - - - - " _ - . L . . . L - _ . o . . - - J 0 6 9 CoagDJat.ioD pH

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3rd Direct Filtration Seminar ."Direct Filtration of Biesbosch Water" 100 ~ ~ 90 ,.;t ~ u ₈₀ q ~ ~ El ₇₀

....

..

~ +

....

~ ₆₀ ~ 10 9 8 7 6 50 I - _ - - ' -_ _. . . L . . . - _ - - L_ _..._ - - - ' 5 COllluJlltloD pH

Fig.18. Effect of coagulation pH on residual manganese in the MJTA filtrate.

3.2.4.2. Application of cationic polymer as a sole coagulant

The application of a cationic polymer as a prime coagulant was considered as an alternative method to validate the established hypothesis and to enhance the removal of the manganese dioxide hydrates. The synthetic cationic polymer, Superfloc C573, selected after preliminary experiments, was used in these experiments. The experiments were conducted with the MJTA under the following conditions: pre-oxidation with potassium permanganate at 0.7 mg/l, coagulation at different dosages of C573, flocculation at constant pH=8.O and filtration through small sca1e filters at the constant filtration rate (1.8*10.3 _{mIs). The effect ofpolymer} dosage on filtrate quality is shown in the Fig. 19 and 20. The graphs show that an increase in polymer dosage, up to approximately 2 mg/l, significantly reduces the partiele count, turbidity and the residual manganese concentrations in filtrate. Further increase in polymer dosage up to 5 mg/l, did not have pronounee effect on filtrate quality. Polymer dosages above 5 mg/l have an adverse effect on residual manganese and partiele concentrations, likely due to charge reversal and partiele restabilisation. The residual manganese concentration in the MJTA filtrate, were below 50 J.'g/l. On the other hand the process applied was not very efficient for turbidity removal. Produced filtrate had unacceptably high turbidity levels. In general, the high polymer dosages required to achieve efficient manganese removal and poor filtrate turbidity preclude the use of cationic polymer as a prime coagulant in real full scale direct filtration applications. Nevertheless, the results from these experiments confirmed that the charge neutralisation is a governing mechanism responsible for the removal of the manganese dioxide hydrates.

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3rd Direct Filtration Seminar 2.0 1 •.1' /.0 0 ..1' , . . . . - - - , 100 - 0 - Tlub. - - PC tfOO o 2

Water-Fig. 19. Effect of the dosage of the cationic polymer on MJTA filtrate turbidity and partiele concentration. ISO ~ ~ J20 ~ ~ ~ _~o I::l ~_I::l QS ₊

El

_1f0

\

/+

~ ~ +-.~ -+ ~ _JO ~ a o I 2 J 4 .1' If 7 I CS 7J dOS81"CmI/J)

Fig.20. Effect of the dosage of the cationic polymer on residual manganese concentration in

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3rd Direct Filtration Seminar "Direct Filtration of Biesbosch Weter"

3.2.4.3. Application of cationic polymer as a coagulant aid

The performed experiments revealed the importance of coagulation with iron salts, to produce filtrate of low turbidity. In addition, it was found that the use of cationic polymer improved partic1e removal and rOOucOO residual manganese. In the next series of experiments an attempt was done to combine the advantages of these two types of coagulants, namely ferric sulphate and Superfloc C573.

Accordingly, the experiments with the MJTA were carried out to determine wheatear the use of combination of iron as main coagulant, at dosage of 1.5 mg Fe(lII)/I and the cationic polymer, Superfloc C-573, at different dosages, as a secondary coagulant or a coagulant aid, could produce filtrate of targeted quality. The other process parameters applied were: pre-oxidation with potassium permanganate at dosage of 0.7mg/l, filtration through small sca1e filter at the constant filtration rate of 1.8*1O"3

m/s.

The results from these experiments are shown in Fig. 21 and 22. The use of combination of ferric sulphate and the cationic polymer at dosages above 0.35 mg/l significantly rOOucOO residual manganese, iron, partic1e concentration and turbidity.Very simple experimental set-up, the MJT A, operating under these conditions produces filtrate of very high quality: turbidity below 0.2 FTU, 99% partiele removal efficiency, residual manganese

<

20 fLg/I

and residual iron

<

0.1 mg/I. These results confirmed earlier assumptions and shown that the direct filtration treatment process supported by pre-oxidation with potassium permanganate and use of combination of ferric sulphate and the cationic polymer could eliminate the potassium permanganate related drawbacks and produce filtrate of targeted quality. The next graph (Fig. 23) shows the partic1e removal efficiency obtained with the MJTA under different treatment conditions. From the graph, it may be clearly seen that the direct filtration supported by pre-treatment with KMn04and application of ferric sulphate and

cationic polymer produces filtrate with the lowest partiele concentration in comparison with other treatment alternatives. Once more it should be noted that a very simple experimental set-up, the MJTAproduces filtrate with the exceptionally low partic1e concentration.

The additional, confirmatory experiments were conducted with the continuous flow experimental set-ups, the BSDFPP and the MDFPP, to verify the results obtained with the batch unit, the MJTA.

The BSDFPP provided a possibility to establish and assess in parallel four different direct filtration treatment trains. The treatment alternatives examined are shown in Fig.24. The common coagulant, ferric sulphate, dosage applied was 1.5 mg Fe(lII)/l; The dosage of polymer, Superfloc C573, was 1.0 mg/l and pre-oxidation with KMn04 at 0.7 mg 11 was

introduced, The filters were equipped with three-layer boos (anthracite/sand/gamet). The constant filtration rate of 10m/hwas applied. The impact of treatment composition on direct filtration performance is shown in Fig. 25, 26, 27 and 28.