Operation of Combined SewersAuthor(s): George E. SymonsSource: Sewage and Industrial Wastes, Vol. 24, No. 4 (Apr., 1952), pp. 533-535Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25031869 .

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Vol. 24, No. 4 1951 OPERATOR'S FORUM 533

on a digester bottom. If it becomes

necessary to clean a digester, however, there are a few points worth consider

ing. The first is means of access to get at the material, be it bottom deposits or scum. The second is a means of

pumping hair balls and similar debris from the digester sump, by a positive displacement pump connecting to the

sump by a pipeline. The third is a

permanent pipeline to a lagoon or other

storage unit for facilitating the di

gester emptying process. Gordon L. Burt, Superintendent of

Sewage Treatment, Portland, Ore.:? The new Portland sewage treatment

plant, which serves a combined sewer

system in an area of almost constant

rainfall, comprises grit removal, meas urement by large concrete Venturi

tubes, primary settling, and separate sludge digestion. No grit has shown

up in the grit channels, which are of conventional design, but about 2 ft. of fine silt has deposited in the Venturi tubes. This silt is being sluiced into the settling tanks, then moved by sew

age ejectors to the digesters, where it no doubt will present an interesting problem in removal.

Despite a wonderful grit removal in stallation consisting of screw conveyors, elevators, etc., we have not yet had any grit?only the fine silt. With this

problem, we are especially aware that it will be increasingly important to

provide sufficient facilities in our di

gesters to remove the accumulated ma terial when necessary.

Alvin A. Appel, Superintendent, Sewer Maintenance Division, Bureau

of Sanitation, Department of Public

Works, Los Angeles, Calif.:?At Los

Angeles we have had few problems as far as grit is concerned. However, I wonder what effect increased use of household garbage grinders will have,

particularly from the settling of coffee

grounds, eggshells, etc., in sewers with

flat grades. There is one point that might be of

interest. We-have 31 pumping plants,

some with quite large wet wells. Re

cently, an operator of an oil refinery became careless, permitting crude oil to enter the sewer and so get to our

pumping plant. Crude oil floated 2 ft. thick on the wet well surface. That

presented quite a problem for removal.

Investigation finally turned up a vac uum pump, which is the heart of the vacuum unit developed to handle such

situations. Briefly, the unit consists of a tank and a vacuum pump mounted on a truck chassis and suitably valved. The operator simply has to lower the suction hose into the well and open the valves. We also have used the unit for

cleaning out all types of clarifiers whose material would be hard to re move in any other manner.

Operation of Combined Sewers

Chairman Symons:?The discussion so far has been on the effect of com

bined sewer operation, because that is what grit largely is. Now, however, we will take up the cause itself ; that is, combined sewer operation.

Mr. Mick:?By way of introducing this subject for discussion, there are

more than 1,600 mi. of sewers tributary to the Minneapolis-St. Paul plant. They are practically all combined; al

though Minneapolis has constructed 212 mi. of separate storm sewers since the plant started operation, probably between 80 and 90 per cent of our

system is still combined. The size of the intercepting sewers

was based on all future extensions

being built on a separate plan. The

designers did not build an interceptor system large enough for the two cities to continue on a combined system in

definitely for all their new areas. That's why they are beginning to build some storm sewers now.

Before the plant could go into oper ation, 52 mi. of interceptors had to be built along both banks of the river to

intercept some 75 old outlets into the river along the 20-mi. stretch through the Twin Cities. Where those inter

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534 SEWAGE AND INDUSTRIAL WASTES April, 1952

ceptors join with the old outlets they are provided with regulators so that in times of heavy storm the excess storm

water overflows directly to the river. The interceptor system was designed on the basis of taking rainfalls up to

0.04 in. per hour without overflowing to the river. Rainfalls of that intensity are only exceeded about 1.7 per cent

of the time, so that actually slightly over 1 per cent of the total sewage flow overflows to the river by reason

of having the combined system. Based on our expected conditions, the

designers allotted 44 per cent of the

interceptor capacity to storm water,

only 22 per cent to the domestic sew

age alone, another 17 per cent to the industrial waste, and 17 per cent to

infiltration. I believe that industrial waste figure is approximately right. Figuring on a strength basis, about 25 to 30 per cent of the plant load is due to industrial waste, which would cor

respond to somewhere near the 17 per cent on a volume basis.

Infiltration is one matter of interest. We've taken several infiltration meas

urements on our interceptor system. At times during the spring we by-pass all of the sewage out of the sewer sys tem so that we can inspect the sewer

and measure the infiltration. We have to be rather careful here with our

quantity measurements, because we

apportion the cost between the two cities on the basis of flow volume. In filtration, is a matter of concern be cause we correct for that in our meas

urements. We do that by placing a

weir in the main interceptor coming into the plant, diverting all the sewage, and measuring the ground-water in

filtration, which has run between 70, 000 and 100,000 g.p.d. per mile of sewer. That value sounds high for

ground-water infiltration, but we da

not consider it excessive for the size

of sewer. (It runs up to nearly 14 ft. in diameter and it is up to 200 ft. beneath the surface of the ground with a head of ground water as high as 40

ft. on the crown of the sewer at some

points.) By walking through the in

terceptor at various times, we've de

termined that most of the infiltration enters at the original construction

joints, although there have been ?

couple of leaks in the sewer itself. In 1944 we discovered one leak about 6

mi. above the plant coming through a

sort of honeycomb section of the con

crete in the invert, about 1 ft. thick and about 12 in. by 18 in. in section. That leak slowly increased, so in 1945

it amounted to 350 g.p.m. and we de cided it should be repaired.

Another leak was discovered about 1.3 mi. above the plant in the spring of 1950. It's in the top of the sewer near

the crown and is only leaking about 50 g.p.m., but we figure we will have to repair that too, probably in another

year.

The sewer was inspected for erosion of concrete and disintegration; prac

tically none has been found in the 13

years we've been using it, except a

slight wear in the bottom 12 or 18 in. of the invert and the two leaks men

tioned.

Deposits in the sewer are always a

matter of interest. We found a few

deposits, principally at points of curva

ture. On the inside of the curve there are deposits of boulders, rags, sand, and grit extending for 100 ft. or more

along the curve, and just below the

curve, up to a depth of 18 in. We cleaned one of those deposits out in

1946, then went back in there again two

years later and found it had re-formed to the same depth. Inspecting it since that time, it does not seem to accumu late to any great extent, and it does not seem to interfere with the use of the sewer at the present time. There are also deposits from some of the St. Paul sections which were formerly creeks; there are a few old lake beds they also drained and naturally a fair amount of boulders wash into those connections and show up just below them.

Our sewer and the plant were both

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Vol. 24, No. 4 TIPS AND QUIPS 535

designed hydraulically for a flow up to

615 m.g.d. and will take that quantity. Nevertheless, from the standpoint of treatment provided and troubles aris

ing from grit and sand, we find that we

can't take more than about 1.5 to 1.75 times our average flow without getting into trouble from flushing sand into the

plant. The grit problems involved have al

ready been discussed. I might mention that we use an alignment chart for

regulating the grit chamber velocities; our operators use that to control the

grit chamber velocity by lining the number of chambers in service against the flow of the sewage so that they can determine what the flow velocity is.

We regard combined sewers as a fine means of freshening the sewage and

cleaning out the system, especially dur

ing hot weather when the sludge is

getting a little on the septic side and the alkalinity is going way up and

making it difficult to filter. Then along comes a heavy rain, which freshens

things up and lowers the alkalinity. But there are disadvantages of grit problems and overflows to the river at the regulators, which have a bad habit of becoming plugged with sticks and branches and sometimes staying open even after the storm is over, so

that they have to be inspected follow

ing each storm. Mr. Bubbis:?What safety precau

tions do you take in inspecting your

interceptor sewer? Mr. Mick:?The usual safety equip

ment is used; that is, the hydrogen sulfide and combustion indicators and the oxygen deficiency lamp, which are used before we enter a sewer. We have never found any dangerous atmosphere in the sewer system during inspection.

There is usually a draft, probably through the old outlets, which are open at the river end where air can still enter.

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