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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
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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.
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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.
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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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