By Da v id P . Ba c k m e y e r
Superintendent of Sewage Treatment, Marion, Ind.
The data and discussion m aterial in this p ap er are divided into two sections or p arts. The first consists of a b rief description of the garbage grin d in g and disposal fac ilities at the M arion sew age treatm ent plant, together with some operation figures which are in tended to illu strate w hat dual disposal has done both to and fo r the treatm ent p lan t d u rin g the past five years. The second p a rt is a b rie f review o f a series o f experim ents th at w ere con
ducted w ith sm all digestion u n its o f 25 gallons each. This latter w ork is not y e t completed, and more inform ation w ill be available fro m it later.
Dual Disposal Experience
A bou t five y ea rs ago the first m otor
ized collection equipm ent w as p u r
chased an d pu t into use b y the C ity G arb age D epartm ent. A s the then re cen tly com pleted sew age treatm ent p lan t w as equipped fo r g rin d in g and disposing o f garbage, city officials at th at tim e w ere anxious th at th is equip
ment be given a f a ir tria l, in an effort to find a s a n ita ry method fo r final disposal o f the garbage. In the month o f M ay, 19 4 1, 18 tons o f green garbage w ere brought to the p lan t fo r g rin d in g as a first test of the fa c ilitie s provided in the o rig in al plan t.
H andling of Garbage
The garb age disposal installation was more th orou ghly tested d u rin g the n ext fe w months. In Septem ber of the
* P re s e n te d a t 18th A n n u a l M eetin g , Cen
tr a l S ta te s Sew age W o rk s A ssn., L a fa y e tte , In d ., J u n e 13-14, 1946.
same year, 16 2 tons o f green garbage were ground and added to the raw sew age at the w et well. The method of h an d lin g the garb age at the plan t was extrem ely sim ple. The garbage was dum ped from the tru cks onto the grade floor and then shoveled into the g rin d er hopper. Tin cans, trash, crockery, pieces of m etal, etc., were sorted out as the garb age w as shoveled.
The ground garbage w as discharged from the Je ffr e y garb age grin d er to the raw sew age w et w ell beyond the sewage screens. The ra w sew age pum ps then discharged the garb age solids w ith the sew age solids to the g rit channel for the rem oval of sew age g rit and garbage m ineral m atter. P reaeratio n tanks at the head end of the p rim a ry tanks a f
fo rded a m eans of releasin g the grease fo r rem oval b y skim m ing at the efflu
ent ends. The m ain b ulk of the garbage solids w as rem oved as sludge w ith the sew age solids, and pum ped to the di
gesters fo r digestion.
T hree definite problem s soon arose from this method o f h an d lin g the gar
bage solids. F ir s t , an excessive amount of tim e w as required fo r draw ing sludge from the p rim a ry tan k sludge hoppers. W hen seven or eight tons of garb age were handled in one day, one operator w ould spend most of the day gettin g the sew age-garbage sludge m ix
tu re into the digesters. Secondly, the load on the secondary treatm ent, which is b y activated sludge, w as foun d to be considerable as a resu lt o f the release o f colloids and solubles from the finely ground garbage. In the th ird place, a larg e amount of floatin g m aterial
Vol. 19, No. 1 D IG E S T IO N O P G A EB A G E W IT H S E W A G E SO L ID S 4 9
F IG U R E 1.— Garbage receiving and mixing well at Marion, Ind., sew age treatment plant.
would get past the grease skimmers and make an u n sigh tly mess when it would collect on the w iers of the final settling tanks. The answ er to these problems seemed to lie in the development of a system in which the garbage could be gotten into the digesters without first m ixin g it w ith the sewage.
In M ay of 19 43, the flooded con
dition of the M ississinew a K iv e r p e r
m itted by-pass of the flow to the tre at
ment plan t fo r a few days, and du ring th is period a portion o f the raw sew
age w et w ell was partition ed off fo r use as a separate garbage w ell. The necessary p ip in g w as installed fo r re
moval o f the garbage, and fo r cleaning out the residue from the bottom of the (w ell. The general plan of the w ell as it is being used today is shown in F ig u re 1 .
Several changes in the method of m ixing the ground garbage have been made in the p ast two years. The agita
tion in the w ell m ust be sufficient to prevent the garbage solids from “ cak
in g ” at the su rface of the accum ula
tion, this trouble being more pro
nounced du rin g the com canning season. I f the ground garbage is left in the w ell fo r a period of 10 to 24 hours, lime must be added to control the p H . Two bags of h yd rated lime (10 0 lb.) are u su ally sufficient to hold the p H above 6.0 when the 3,600-gal.
batch is not pum ped to the digesters fo r a period of 18 to 24 hours.
Gas Production and Utilization I f the process o f disposal o f garbage b y digestion with sewage solids is to be an economical one, some practical
50
S E W A G E W O R K S JO U R N A L J a n u a r y , 1947 use m ust be made of the gas produced.A stu d y of the data on hand at M arion w ould indicate th at an average y ield of 260 lb. o f volatile solids from a ton o f green garbage should produce 2,600 cu. ft. o f gas h avin g a carbon dioxide content of less than 35 per cent b y volume.
D u rin g the y e a r 19 4 5 the M arion d i
gesters produced 23.6 m illion cubic feet of gas, and utilized 2 1.5 m illion cubic feet at the gas engines and heat
in g boiler. In addition to the sewage gas used, 640,000 cubic feet of gas was purchased from the u tility com pany to fill in at certain periods when gas p ro duction w as not sufficient fo r the total p lan t load.
S eve ral in terestin g studies have been m ade in an effort to determ ine w hat percentage of the total gas gen
erated comes from the sewage solids and w hat p a rt from the garbage solids.
Two methods are available fo r a p proach to this problem in .actual plan t operation. I f the sewage load is stable, th at is, not too f a r from average w ith respect to B .O .D . strength as a result
o f ra in w ater dilution or in d u stria l wastes, the garb age could be taken a w a y fo r a fe w d ays and the resu ltan t drop in gas production noted. B y w o rkin g in reverse order, i f the garbage load w ere stable an d the sew age load w as reduced b y prolonged rain s, some indication would be had as to the am ount o f gas gen erated from the g a r
bage alone.
The w in ter months afford an oppor
tu n ity to stu d y gas production volumes when the am ount o f garb age handled is v e ry lim ited. F ig u re 2 dem onstrates the fluctuation in gas production throughout the calen d ar y e a r. The curves represen t the average o f fo u r y e a rs operation b y sin gle months. It w ill be noted th at d u rin g the first qu ar
ter o f the yea r, when the garb ag e ton
n age is below two tons p e r day, the am ount o f gas produced is less than 49,000 cu. ft. p er day. In the months o f J u l y , A u g u st, and Septem ber, the gas y ield increases about 30 p e r cent w hile the am ount of garb age ground increases 400 p er cent. I t is the w r ite r ’s b elief th at more gas w ould be
A V G . B Y M O N T H S O F F O U R Y E A R S G A R B A G E L O A D I N O A N D G A S P R O D U C T I O N 1 9 4 2 4 3 4 4 . 4 5
F IG U R E 2.— Four-year record of gas production.
Vol. 19, N o. 1 D IG E S T IO N O F G A R B A G E W IT H S E W A G E S O L ID S
51
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M O N T H S OF Y E A R 1944
F IG U R E 3.— Relation between garbage load and gas production.
collected in the summer months if more digestion cap acity were provided.
F ig u re 3, which is reprin ted from the fo u rth annual report of operations, shows the relationship of the various factors affecting the production of gas du rin g the y ea r 1944. H ere the ra in fa ll curve and the canning p lan t load curve are included, as both of these factors have a definite bearin g on the sewage organic load.
F ig u re 4 is intended to show the sag in gas y ield when rain w ater washes out the gas-producing sewage solids, and the garbage solids m ust produce most of the gas. The B.O .D . curve
indicates the drop in organic m atter in the sewage, w ith the resultant drop in gas volume from 49,000 to 26,000 cu. ft. p er day. H ad this “ w ashout”
occurred in J u ly , the actual drop would have been about the same, but the range would have been from 75,000 to 52,000 cu. ft. per day. The greatest volume of gas produced in any one single day was recorded in Septem ber, 19 45, when 97,108 cu. ft. were metered.
In A ugu st, 1946 it was possible to stu dy an 1 1 -day period du rin g which time no garbage was taken at the plant.
The average d aily gas production in this period was 23 per cent below the
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S E W A G E W O R K S JO U R N A L J a n u a r y , 1947F IG U R E 4.— Effect of sew age “w ashout” on gas production.