sewer inspection experience

Sewer Inspection ExperienceAuthor(s): Ronald HicksSource: Sewage and Industrial Wastes, Vol. 29, No. 2 (Feb., 1957), pp. 230-233Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25033285 .

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230 SEWAGE AND INDUSTRIAL WASTES February, 1957

difficulty was encountered with plugged

sludge draw-off lines due to an ac

cumulation of sand and grit in the

sludge hoppers. The plug valves are

badly worn and will be replaced. The four aerators and three of the

four final clarifiers operated continu

ously. The practice of blowing the

aeration plates two or three times each

year was continued with satisfactory results. The gear reduction box on

one of the clarifiers was sent to the

factory for repairs.

Sludge Digestion

Normal operation of the six digesters as two three-stage units was continued. The reduction in gas produced per

pound of volatile solids digested has

been evident for 8 to 10 years. During the winter months there was not suffi

cient gas to maintain a comfortable

working temperature in the buildings and also to keep the digester tempera tures sufficiently high to facilitate di

gestion. Continuation of this condi

tion, which at present is attributed to

the increased use of detergents, will

necessitate the purchase of auxiliary fuel. Future plans call for the de

watering and cleaning of the two first

stage digesters. The original mono-rail system used

to remove sludge from the drying beds was sold as scrap. Reinforced concrete

ribbon drives through each bed were

constructed to replace the mono-rail.

Cleaning of the beds can be accom

plished just as quickly and there will

be a considerable saving in mantenance over the years. The change-over cost

only about iy2 times the cost of one

painting of the mono-rail system.

Plant Maintenance

The program of preventive mainte nance has been largely responsible for the continued uninterrupted operation of the treatment units.

The 5-ft. high chain link fence en

closing the plant grounds was taken down and regalvanized. The posts and top rails were painted with a rust

inhibitor and the fence re-erected. All

work, except taking the fence down, was done on contract. The total cost was approximately l1/^ times the cost of painting with one coat of rust in

hibitor paint. Use of the plant grounds for picnics

and recreational activities has in

creased. Additional picnic benches

and stoves were purchased and placed on the 35 acres of plant park.

Table II summarizes the operation data for the fiscal year 1954-55.

SEWER INSPECTION EXPERIENCE

By Ronald Hicks

Chief Chemist, Auckland Metropolitan Drainage Board, Auckland, New Zealand

The Auckland Metropolitan Drain

age Board is responsible for all main sew?rs and sewage treatment works

within a metropolitan district cover

ing some 200 square miles. At the

present time, a population equivalent

Editor's Note?The author took the

photographs for this article with an in

expensive 2*4 -in. by 2%-in. Voightlander camera with flash.

approaching 200,000 is served by a main sewer nearly seven miles in

length. The sewage passes through bar screens and some grit is removed, following which it is discharged to

Waitemata Harbour. A comprehen sive sewerage scheme is now under con

struction to serve a population equiv alent of about 1,500,000 by the year 2000. It is programmed to bring the new scheme into use by 1960, when



Vol. 29, No. 2 SEWER INSPECTION EXPERIENCE 231

FIGURE 1.?The way down!

the present discharge into the Waite mata Harbour will cease. The exist

ing main sewer will, however, be re

tained as a trunk main serving the main city area.

The main sewer was constructed be

tween the years 1909 and 1912. It is

egg-shaped for the whole of its length and its dimensions vary from 8.5 ft.

by 5.76 ft. at the discharge end to 5 ft.

by 3.34 ft. at the upper end. The

gradient decreases toward the dis

charge end from 1:1,700 to 1: 3,000. For various reasons, detailed inspec

tion of this sewer had not been made

FIGURE 2.?Variation in grit deposits.

FIGURE 3.?Miscellaneous articles (silver ware and coins) found in the sewer.

for many years. By 1953 it became evident that silting was taking place in some sections and that attention

was urgently needed. The sewer for

the most part serves an area of com

bined sewers, with the result that fre

quent surcharging occurs in wet

weather. The rapidity of onset and

high intensity of local rains result in

a rapid increase in flow without much

warning. Rainfall is fairly evenly

spread throughout the year and no

reasonable prediction of "dry weather

periods'' can be made. Furthermore,

high hydrogen sulfide intensities are

FIGURE 4.?The start of root penetration.



232 SEWAGE AND INDUSTRIAL WASTES February, 1957

FIGURE 5.?Damage to sewer caused by root growth.

encountered in various parts of the sewer. Flushing from a large tannery,

using sodium sulfide as a delapitory, has been practiced for about 30 years at a point 30,000 ft. from the outfall, so that insoluble sulfides were prob ably mixed with the silt along most of the sewer length.

One of the main difficulties to arise was that access to the sewer was most

difficult, manholes being few and far between. For example, between the

lengths 23,700 ft. and 35,000 ft., only seven manholes were available, vary

ing in depth between 27 and 56 ft. Of these seven, three did not give direct access to the sewer but led down via side adits. The diameter of the

manholes was only 19.5 in.

The first man entering the sewer

for inspection was equipped with a

fully self-contained oxygen breathing

apparatus and was capable of full im

mersion in water if necessary for about one-half hour. This leader was at

tached to a stout life line and carried a safety lamp capable of showing the

presence of the normal toxic volatile

compounds likely to be encountered in

sewer air. A rough measurement of

the hydrogen sulfide content was ob

tained by the exposure of large filter

papers which had the word "Danger''

written on them with a paint brush

using either a lead acetate or sodium

plumbite solution. The rate of ap pearance of the word in readable form and the deepening in intensity of the letters against the white background of the otherwise unmarked paper, be came a guide to indicate the hydrogen sulfide concentration. Psychologically the rapid appearance of the ''Danger'' sign, readable even under very poor

light conditions, proved very effective and none of the personnel concerned was ever inclined to ignore it.

Even where direct access was avail

able, great care was needed in descend

ing the manholes. Figure 1 shows a

typical manhole, 43 ft. deep, with

every step greasy. Normally a grat ing is positioned between the two sets of rungs, but this had to be removed to permit rubble being drawn up in baskets.

Deposition of grit proved quite troublesome; as shown in Figure 2

sudden changes in the amount of grit on the bottom of the sewer frequently led to complete immersion of the in

spector. In a distance of 36 ft. the

depth of silt varied from 3 ft. to 3 in.

The bricks and rubble buried in the fine silt presented a constant obstacle, especially for the unwary, and many a

FIGURE 6.?Complete destruction of the

9-in. by 9-in. concrete sewer blocks.



Vol. 29, No. 2 TIPS AND QUIPS 233

toe was stubbed. This rubble was

lifted out of the sewer by manpower and heaped on the street above the

manhole. An interesting collection of miscellaneous articles found among this rubble is shown in Figure 3.

Inspection showed that tree roots, obviously in search of water, had pene trated through the concrete slabs in

several lengths of the sewer. The com

mencement of root growth is shown in

Figure 4, and the rapid breakdown of otherwise good concrete when the root has a firm hold is shown in

Figure 5. Figure 6 shows how com

plete destruction of the sewer took

place, resulting in damage so serious that a new arched roof had to be

provided over a length of approxi mately 40 ft. Some sewer sections constructed with concrete blocks were

in excellent condition and no root

penetration was observed.

In addition to immediate local re

pairs which are now being given at

tention, nearly 6 miles of sewer will need to be freed from heavy accumu

lations of grit.

TIPS AND QUIPS

Froth vs Foam

From time to time reference is made to the frothing of aeration tanks rather than foaming, although the two words are essentially synonymous in diction

ary definition. The term "froth" has

purposely been adopted by This Jour nal (24, 5, 670; May, 1952) in re

ferring to the aeration tank problem as

distinguished from the "foam" prob lem at sludge digestion units.

Progress Against Pollution

The Ohio Eiver Valley Water Sani tation Commission reports that now 85

per cent of the 3,600,000 population adjacent to the banks of the Ohio have

sewage treatment plants in operation or under construction. Thus the Ohio

Valley has progressed from three out of every eight persons served by sewers

in 1948 to six out of eight in 1956.

Although much remains to be done in this field, the progress has been

encouraging. The evidence is summed

up in the statistics reported by Engi neer News-Record which show more

than 50 per cent increase in sewage works construction volume in 1956 over

1955. And this, without the newly au

thorized federal aid.

Automation

The U. S. Bureau of the Census re

ports that the farm tractor has re

placed the horse, and that more farms have tractors than have running water.

However, Silas is still using the old

path.

Basic Sewage Chemistry

Information, in simple and under standable terms, to help the sewage plant operator in his laboratory will be found in the article "Water and

Sewage Chemistry and Chemicals, "

Public Works, October 1956.* If you haven't already appropriated those

pages for your ready reference or per

sonal refresher course, it is recom

mended that you get a copy soon. This revised and expanded article by Kenneth W. Cosens, Associate Profes sor of Sanitary Engineering, Ohio State University, uses color so that cer tain colorimetric tests are illustrated as clearly as an actual laboratory dem onstration.

Company Safety Booklet

A new booklet, designed to sum marize a company safety program for

Public Works Journal Corp., 200 South Broad St., Ridgewood, N. J.



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