107
Steef de Valk, MSc
Steef de Valk TU Delft
The enzymatic degradation of
excess activated sludge: A tale of
worms
Introduction
The activated sludge process is the most used process to remove organic carbon, nutrients and other pollutants from sewage and also from many industrial waste waters. The organic fraction of waste water is aerobically respired and partly converted into biomass. The surplus biomass is a by-product of this process and is called excess activated sludge. The main constituents of activated sludge are biomass, organic matter and water. In general, this sludge stream is partly converted in biogas upon anaerobic digestion and partly processed e.g. dewatered and incinerated. One of the drawbacks of the activated sludge technology, is the cost for processing and disposal of the large amounts of excess sludge.
The major fraction of excess activated sludge consists of complex organic matter, which could be utilized if transformed into VFA precursors for use in (bio)-chemical industrial processes or biogas. Hereby increasing the valorisation of sludge and reducing the amount of sludge associated with further processing and disposal costs. The problem with increasing valorisation lies in the rate limiting step: Hydrolysis of complex organic residues.
In order to reduce excess activated sludge production, Lyse-cryptic growth technologies, such as the Cannibal process, are considered, as well as predation by aquatic worms. In order to improve dewaterability and accelerating anaerobic digestion of the excess sludge, and in that manner also reduce the total amount, pre-treatment methods are researched, such as ozonation, sonification, thermo and pressure processes, and enzyme dosing which increase the hydrolysation rate of the sludge.
Apart from sludge reduction, interest for maximising the sludge energy potential and thus enhancing the rate limiting hydrolysis step is also increasing. Studies are ranging from localization and characterization of hydrolytic enzymes present in anaerobic digestion, microbial community characterization, sludge characteristics (e.g. the presence of inhibiting compounds, particle size, settleability), to describing the digestion process in kinetic models and reactor design.
108
Vakantiecursus 2013
The enzymatic degradation of excess activated sludge: A tale of worms
Aquatic sludge reducing worms
“Without the work of this humble creature, who knows nothing of the benefits he confers upon mankind, agriculture, as we know it, would be very difficult, if not wholly impossible”
-Charles Darwin The aquatic worms that naturally inhabit waste water treatment plants have gained increasing attention. The worm predation method is a special case of surplus sludge reduction. This method is based upon the ecological principle that energy is lost when passing from lower to higher trophic levels. Aquatic worms such as Tubefix tubefix, Lumbriculus variegatus, Aulophorous furcatus and Limnodrilus hoffmeisteri have been investigated and implemented in new reactor designs in order to reduce the amount of sludge formed and/or reduce the amount of excess sludge (Hendrickx et al., 2009; Tamis et al., 2011; Guo. X et al., 2006; Tian. Y 2012). Sludge reductions of 8-40% (TSS based) have been reported depending on the type of worm and the experimental setup used. More specific, Tamis et al., 2011 reported that the aquatic worm Aulophorous furcatus, increased the biodegradability of excess sludge significantly, in terms of process time (SRT 2 days) and methane potential (Worm predated sludge showed the same BMP as regular excess sludge even though part of the sludge was already consumed by the worms), compared to other sludge reduction methods currently available.
Worms feed on the complex bio matter present in sludge by hydrolysing (part of) the polymeric substances present (e.g. biomass, Extracellular Polymeric Substances (EPS) and possible other polymeric compounds) There are numerous publications on the implementation of aquatic worms dealing with sludge reduction and even with reducing membrane fouling (Tian et al,. 2012). However research concerning the actual mechanisms behind the reduction of sludge is minimal.
Enhanced Enzymatic Anaerobic fermentation of organic residues (EnzyFOR)
The EnzyFOR project is a collaboration between the technical university of Delft, Wageningen UR, STW, STOWA and industrial and consultant partners: DSM, Delfluent Services BV, Royal Cosun and consultant and engineering firm Tauw.
The aim of the EnzyFOR project is to increase the valorisation of excess activated sludge by developing a cost effective process for the degradation of complex organics into VFA or biogas. The research team in Wageningen UR has its focus on agricultural residues. The research in Delft focuses on the explanation of the increased sludge biodegradability, observed in worm predated excess activated sludge. In Wageningen research is carried out by Samet Azman, supervised by G. Zeeman, A. Stams and C. Plugge. The TU Delft research is performed by the author and the supervisory team consists of J. van Lier, M. de Kreuk and M. van Loosdrecht. Ahmed Khadem has a post-doc position in the EnzyFOR project. His task is to research in detail the role of natural organic matter in hydrolysis and to link the research performed in Wageningen and Delft.
Project outline
The research in Delft will give insight into ways to mimic the biological activity of aquatic sludge degrading worms for large scale processes. The aquatic worm Aulophorus furcatus will be the first predating worm that will be studied in this research project. For this purpose a lab scale worm reactor is operated at the sewage treatment plant Harnaschpolder, Delft, The Netherlands.
109
Steef de Valk, MSc
In order to mimic the biological activity of worms on the degradation of excess sludge, the first step is to identify what part of the sludge the worms consume. Aquatic worms have been shown to selectively feed on sludge. Particle size and bacterial species preferences depend on the type of worm. (Inamori et al., 1987, 1990; Kuniyasu et al., 1997) Protein components of the sludge are selectively consumed (Hendrickx et al,. 2009; Tian et al., 2012) It is unknown whether aquatic worms degrade hydrolysis inhibiting compounds present in waste water such as humic and fulvic acids or what parts they consume of the extracellular polymeric substances (EPS) present in excess activated sludge. To shed more light on substrate preference, worm predated sludge will be compared to untreated sludge, both processed under the same conditions in order to make a distinction between operation conditions (e.g. aeration and the associated bacterial activity) and the activity of the worms.
The second step is to pinpoint the hydrolytic activity of the sludge degrading worm. The biological activity is comprised of the mechanical and biological activity of the worm itself and the activity of the microbial community present in the intestines of the worm. The identification of the part of the sludge that worms consume will yield targets for metagenomic analysis of the microbial community present in the intestines of the worms and the worm itself.
The results obtained during the first and second step will help to mimic the conditions of the worm track. The final goal is to develop a technology to enhance the hydrolysis step of excess sludge digestion in order to increase the biogas production and/or the production of VFA from waste.
Figure 2 - Tubefix tubefix worms on carrier material. Figure 3 - Tubefix tubefix in pilot worm reactor at WWTP Wolvega.
