present status of sludge gas utilization

Present Status of Sludge Gas UtilizationAuthor(s): Richard H. GouldSource: Sewage Works Journal, Vol. 19, No. 2 (Mar., 1947), pp. 170-174Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25030437 .

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PRESENT STATUS OF SLUDGE GAS UTILIZATION *

By Richard H. Gould

Director, Division of Engineering and Architecture, Dept. of Public Works, New York City

For a comprehensive discussion of

the status of sludge gas utilization it

would be necessary to canvass the ex

periences of literally hundreds of im

portant installations in this country and abroad where sewage sludge is di

gested and the resulting gas is collected

and utilized. Rather than attempting such an arduous and time consuming task, the present contribution will be confined to the experiences in the sew

age works of New York City during the past ten years.

It is important to bear in mind that

sludge digestion and gas utilization is but one of several alternate methods that may be available in the disposal of sewage solids. Its adaptability and

economic feasibility will vary consid

erably in different localities and with

special conditions that always are pres ent in each particular project.

Much depends on the possibilities of

disposal of the ultimate residues. If

disposal at sea is available there is one set of circumstances. If large tracts of cheap land may be had the possibili ties are different. Fertilizer manufac ture may be attractive to some, while incineration may be the best compro

mise to others. A determinant in

many cases will be the economic avail

ability of possible treatment plant sites, their size and degree of isola tion.

In New York City in recent years the trend has been definitely toward the digestion of sludge and the utiliza tion to the extent considered practical of the resulting gases. New York

City's largest plant, on Wards Island, *

Presented at Nineteenth Annual Meeting, Federation of Sewage Works Association, To

ronto, Canada, October 7-9, 1946.

and the first of its modern plants to be

placed under construction, is the only exception. Here the solids removed from the sewage are shipped to sea in a fluid state with no treatment other than the d?cantation of excess liquids.

Of the newer plants in operation all

provide for sludge digestion. At

Coney Island, Tallmans Island, Bow

ery Bay and Jamaica the digester gas is utilized to generate power and heat, and at one small plant (City Island) the gases are used for heat only. The same condition will obtain in the 26th

Ward plant now under construction and those for which plans have been

made or are under way. Included in the latter category is the extension of the present Wards Island plant to over twice its present capacity.

Conditions Favoring Digestion in New York Plants

It may be of general interest to out line some of the conditions that make

sludge digestion adaptable to the New York problem.

Of the seventeen locations in use or

adopted for future plants there are

only four that are distant as much as

1,000 ft. from residential areas; many are much closer, hence odor control

and aesthetics are of paramount im

portance. Control of large scale fer tilizer production has not yet been de

veloped to a stage where it could be

safely practiced in most of these loca tions.

One distinct asset is the city's loca tion on the ocean and the economy, simplicity and lack of offense with

which sewage solids can be disposed of at sea. The disposal of the voluminous raw solids from the activated sludge

170



Vol. 19, No. 2 PRESENT STATUS OF SLUDGE GAS UTILIZATION 171

plant at Wards Island is accomplished at an operating cost with city-owned vessels of about 12 cents per ton of

liquid sludge, or about $2.64 per ton of dry solids. This is, of course, much

cheaper than local costs would be for

filtration and incineration.

There is a problem, however, in the

control of odors at the plant site in the

storage and loading of the raw sludge on the vessels. Provision has been

made for the use of ozone in the sludge storage building and for the use of activated carbon at the vents of the

sludge tanks on the vessels, but the

latter was found difficult to maintain in effective condition and has been dis

continued. In general, however, no

serious trouble from odors has de

veloped.

It has been found that when the

combined primary and activated

sludges are digested the destruction of

organic matter and the greater concen

tration of the solids result in a volume

of residue of about one-half the bulk

of the raw sludges. The resulting

saving in the shipping cost of the resi dues is enough to pay for the capital and differential operating costs of di

gestion tanks. Digestion tanks can be

operated without offense. The hazards of handling raw sludges are thus

avoided and at no additional cost. There is substantial value in the large volumes of methane gas that are col lected as a by-product of digestion. It is the utilization of this by-product, secured at substantially no additional

cost, that is the subject of the present discussion.

Gas Utilization in New York Plants

Coney Island

New York City first embarked on the

digestion of sludge and the utilization of digester gases at the Coney Island

plant in 1936. The first installation here was designed for a flow of 35

m.g.d. This plant was laid out to pro vide chemical precipitation and chlo

rination during the summer and plain sedimentation only during the winter.

Eight digestion tanks with gas holder

type covers of our own design were

provided. What was then considered a bold

step was taken in that entire reliance for the power requirements for pump

ing and all other uses was placed on

three 300-h.p. gas engine generator

sets. No provision was made for the

purchase of electricity from an outside

source, however, provision was made

for the purchase of utility gas in case

of a possible deficiency of digester gas. In 1941 the capacity of the plant

was doubled. Six new digesters were

added. These were fitted with covers

floating on the sludge, as the gas stor

age in the older plant was considered

adequate. Three gas engine generator units, each of 920 h.p., were added. The digestion tank capacity now totals

1,200,000 cu. ft., which is 2 cu. ft. per

capita on the design population of

600,000. Gas storage amounts to

400,000 cubic feet. Gas utilization at the Coney Island

plant has been eminently satisfactory. Sewage has been lifted 40 ft. and aux

iliaries and services maintained with but a trivial amount of utility gas being purchased. There is no means to record surplus gas wasted, but gas utilized and metered represents less than half of that which should be re covered from the volatile matter added to the digesters, based on experience in other city plants. In ten years of

operation there has been no flooding of the sewers due to power failure.

Heat recovered from engine cooling water and exhaust gases has been ade

quate for heating the digestion tanks and plant buildings except on a few

extremely cold days when digester gas was used under the heating boilers.

Tallmans Island

The second digester installation to go into service was at Tallmans Island in 1939. This plant of 40 m.g.d. ca



172 SEWAGE WOEKS JOUENAL March, 1947

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FIGURE 1.?Gas engines driving pumps and compressors at Tallmans Island sewage treatment plant, New York City.

pacity is of the activated sludge type. It was felt that the power requirements of this process would exceed the poten tial power that might be secured from

gas utilization. The four digestion tanks, providing 705,000 cu. ft. ca

pacity, were therefore fitted with float

ing covers and no gas storage was pro

vided as little excess gas over the de

mand was expected.

Four gas engines were directly con

nected to positive displacement blowers for the air supply and four others by

means of angle gears to the main verti

cal centrifugal pumps working against a 39-ft. head. Facilities were provided to make up any deficiency in gas pro duction by utility gas. Electric power for lighting and auxiliary equipment

was purchased from the power com

pany. The eight engines have total

rating of 3,620 horse-power.

Gas production at Tallmans Island

has more nearly met power require

ments than was expected. There are

many days when full engine require ments are met and no gas is purchased.

The annual average for 1945, for ex

ample, shows that of the gas produced 3 per cent was wasted and the re

mainder met 91 per cent of the engine demand. About 1.4 per cent of all gas was used for heat during cold weather.

Taking into account the electric power

purchased for auxiliaries as well as the

requirements of the main pumps and

blowers, the gas produced accounted

for a little over 75 per cent of the total

power requirement for the plant and all but a very small part of the heating of digesters and buildings.

Bowery Bay

The 40 m.g.d. activated sludge plant at Bowery Bay was placed in full op eration in 1942. Four digestion tanks

provided 728,000 cu. ft. capacity for a

population of 308,000. They were

fitted with gas holder type covers giv

ing 206,000 cu. ft. of storage.



Vol. 19, No. 2 PRESENT STATUS OF SLUDGE GAS UTILIZATION 173

Negotiations with the utility com

pany for outside services were not en

tirely satisfactory. It was not practi cable to bring in a supplementary source of gas and the company opposed cross connections on the electrical

boards that would allow alternate op eration of the main pumping and

blower units either by generated or

purchased power. This has resulted in a lack of flexibility and some unbalance in the proportioning of the capacity of

units on the gas engine half of the

plant.

Two 800-h.p. gas engine generating units were installed. While in some

months they may produce nearly 70

per cent of the total power required at

the plant, the annual averages show

only about 45 per cent of this require ment. About 7 per cent of the gas

produced was used under the heating boilers.

Jamaica

The 65 m.g.d. activated sludge plant at Jamaica started operation in 1943. This plant designed for 500,000 people, has twelve digestion tanks of 1,618,000 cu. ft. capacity with gas holder covers

of 400,000 cu. ft. capacity. There are

three gas engine generator units each of 1,450 b.h.p.

The arrangements with the utility company at this plant are much more

satisfactory than at Bowery Bay. Here there is a utility gas connection to supplement the digester gas if neces

sary. The main pumping and blower units may be supplied with either pur chased electricity or that generated from gas.

Gas production here was sufficient in 1944 and 1945 to supply 85 per cent

of all the power used and on some days there has been a surplus. It should be

noted, however, that for the period in

question the plant was operating with low solids in the aeration tanks under the system developed and described under the term "modified sewage aera

tion." Air consumption has been

about 0.4 cu. ft. per gal. of sewage. Heat requirements for the digesters and buildings were met for the most

part by heat recovered from engine operation. Only about 1 per cent of the total gas produced and purchased

was burned under the heating boilers.

Design Practice

There is nothing particularly unique in the New York City sludge digestion

practice. In the four plants described above the tanks are circular, varying in diameter from 54 ft. in the early Coney Island plant to around 80 ft. for the others. They have conical bot toms with side water depths from 24 to 30 feet. Temperatures are main tained at from 85? to 90? F. by means

of pipe coils in the tanks. The hot water circulating therein comes from the water jackets of the engines, the heat exchangers from the engine gases and may be supplemented by gas or oil fired boilers. Part of the tanks are

used as primary and others as secon

dary digesters with surplus liquid de canted from the secondaries.

The new designs provide for larger tanks. In some cases diameters will be as much as 120 ft. and side water

depths up to 40 feet. In these larger tanks multiple inlets will be provided to assure distribution and mixing of solids in the tanks. Provisions for

positive circulation are also to be made. At the proposed Hunts Point plant preheating of sludge before discharge to the digesters is called for.

The quality of the digester gas is

quite uniform, averaging about 64.4

per cent methane with plus or minus variations of from 2 to 3 per cent. Its heat content is about 654 B.t.u. as com

pared to the local utility gas of about 540 B.t.u. The sulfur content aver

ages about 2.4 gr. per 100 cu. ft. and has not required any special treatment.

Dual Fuel Engine at Tallmans Island

The engines are of the heavy duty slow speed type. Many of them are



174 SEWAGE WORKS JOURNAL March, 1947

designed so that they can be rather

easily converted to the diesel cycle. In

deed an interesting recent development is based on the conversion of one of the

blower engines at the Tallmans Island

plant to the full diesel cycle and its

operation with oil and digester gas as

fuel.

The engine at Tallmans Island has

been under test operation for over a

year. It is known, of course, that the

diesel cycle permits approximately 25

per cent greater power recovery from a gaseous fuel as compared to the Otto

cycle of the ordinary gas engine. It

has been found elsewhere that in order

to ignite a gas uniformly at diesel

pressures it was necessary to inject a

small amount of oil with the gas. It was found that successful operation with digester gas was possible with the

injection of diesel oil in the amount of

from 5 to 7 per cent of the heat value

of the total fuel used. The engine per formed well with the use of 100 per cent oil or any combination of oil and

gas between these limits. Economies

in fuel consumption were demonstrated to be quite as anticipated.

This new combination seems to offer

great advantages in the utilization of

digester gases. The greater economy of the diesel cycle will make the gas

go further and in most cases would

seem to stretch the supply so that all

normal power requirements can be met.

Should there be a deficiency of gas dur

ing the first months of operation, or

later for any reason, the cost of power

using oil alone as a fuel would be less

than power from purchased gas or

from electricity that is subject to the

usual demand charges. The possibilities of this new combina

tion are so promising that one existing contract for gas engines for the 26th

Ward plant has been modified to con

vert these engines to the dual fuel Diesel type. Provision is being made in new plants under design for installa

tion of this new type. In these new

units the supercharging of gas fed to

the engines will probably be provided for, thus further increasing the fuel

economy and increasing the capacity of units of equivalent size.

Conclusion

The utilization of sludge gas does

undoubtedly call for considerable extra effort in plant design and operation. To be successful it presupposes intelli

gent layout and proper operation and

maintenance. With a power installa

tion that is reasonably well loaded so

that the capital charges are not out of

line there is a very substantial margin of cost in favor of the utilization of this

by-product of sludge digestion.

SLUDGE GAS UTILIZATION?A DISCUSSION

By A. M. Boehm

Engine Division, Worthington Pump and Machinery Corp., New York, N. Y.

Since Mr. Gould's very informative

paper covers utilization of sewage

sludge gas in many ways, some of

which are foreign to the writer's field, these remarks will be confined to that

portion which deals with gas utilization

for power purposes and, more spe

cifically, to the dual fuel engine. Perhaps it would not be amiss at this

time to define a dual fuel engine as it

is now understood. Briefly, it is an

internal combustion engine which con

sumes gaseous fuels, using compression

pressures approximating those of a full

Diesel oil engine and employing a small

amount of fuel oil called "pilot oil"

to ignite the compressed gas and air

charge.



present status of sludge gas utilization

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