Sludge Disposal Practices in the Pacific Northwest

Sludge Disposal Practices in the Pacific NorthwestAuthor(s): Robert E. LeaverSource: Sewage and Industrial Wastes, Vol. 28, No. 3 (Mar., 1956), pp. 323-328Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25033026 .

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THE OPERATOR'S CORNER Conducted by Donald P. Schiesswohl

SLUDGE DISPOSAL PRACTICES IN THE PACIFIC NORTHWEST *

By Robert E. Leaver

Senior Public Health Engineer, Washington State Department of Health,

Seattle, Wash.

The disposal of sludge from a sew

age treatment plant receives operator comment whether the plant is large or

small. Disposal methods vary from the small treatment plant with an an

nual drying of sludge on the drying beds to the large treatment facility located on the seacoast, where sludge is barged to sea in special tankers.

The sludge disposed of may be raw or digested; a liquid or a dried solid.

When digested its public health sig nificance is less critical and, when

properly treated, dried sludge is as

safe to handle as a commercial ferti lizer.

At the majority of sewage treatment

plants sludge disposal is a problem to some degree and it is a necessary op

erating expense at all treatment fa cilities. Methods of sludge disposal and the financial burden entailed were

studied for several municipalities in

the Pacific Northwest, principally in

the state of Washington.

Comparative Value of Fertilizers

To better understand the problem involved in the disposal of sludge it is

well to consider the fertilizer value of

* Presented at the 1955 Annual Meeting,

Pacific Northwest Sewage and Industrial

Wastes Assn.; Victoria, B. C; Oct. 27-29, 1955.

available organic fertilizers and their cost. The fertilizing ingredients in

sewage sludges, various manures and

organic nitrogenous material are pre sented in Table I. Digested sludge, in

general, is about equivalent in fertiliz

ing value to manure. In Seattle, Wash., manure costs approximately

$6.50 per yard or $16.60 per ton, based on a dry weight of 1,300 lb. per yard and a 60 per cent moisture content.

Humus sells for $2.00 per yard when

picked up at the pit, or approximately $5.10 per ton on a dry weight basis. The heat dried activated sludge, Mil

organite, sells for $4.55 in 80 lb. sacks or the equivalent of $114 per ton, dry

weight. The major portion of this cost is due to transportation charges. The

Milorganite returns $33.64 per ton,

wholesale, at the treatment plant in

Milwaukee, Wis.

Sludge Disposal Methods

The principal methods of digested

sludge disposal in the state of Wash

ington and adjacent states utilize

sludge drying beds, tank trucks, vacuum filters and lagooning. The heat

drying of sludge is not generally

practiced nor are there any installa tions incinerating the sludge as a

method of disposal. Most of the sludge 323



324 SEWAGE AND INDUSTRIAL WASTES March, 1956

TABLE I.?Fertilizing Ingredients in Sewage Sludges, Various Manures and Organic Nitrogenous Materials

Material Fertilizing Ingredient1 (%)

Nitrogen (N) Phosphoric Acid Potash Organic Matter

Digested settled sludge Digested settled sludge with"trickling

filter sludge

Digested activated sludge Heat dried-activated sludge Commercial pulverized

Sheep manure

Cattle manure

Poultry manure

Animal tankage Blood Fish scrap Cottonseed meal

Castor pomace

0.8-3.5

1.0-4.5

2.0-4.8

4.0-7.0

1.2-2.5

1.6-2.1

1.9-4.0

5-10

9-13

6.5-10

5-8

5-6

0.7-4.02

tr.-4.02

1.0-3.62

1.7-2.5

1.0-2.0

1.0

2.5-3.7

7-16

0.5-14

5-10

2-3

2

0.8-1.62

0.13

2.0-4.0

1.0-2.2

0.8-1.3

1-2

1

30-603'<

48

66

64

^eeref. 1. 2 Dry basis. 3 Estimated from other tables. 4 Volatile matter.

is given away to anyone who will haul it from the drying or dump area.

Liquid Sludge

Liquid digested sludge is usually re

moved from the sewage treatment

plant by private contractor or mu

nicipal tank truck. The following ex

amples are typical of current practice. Olympia, Wash., population 17,000,

has a contract with a nurseryman and receives approximately $4.80 per ton on a dry weight basis. The contractor

gets the liquid digested sludge at the treatment plant in his own tank truck for $2.00 per 1,000 gal. load. The contract requires that the nurseryman take the total annual production of

sludge as it becomes digested (less than 40 per cent volatile solids). The con

tractor apparently is well pleased with

the arrangement and the results he

gets from spreading the sludge on land

used for growing shrubs and other

nursery stock. A five-mile round trip is necessary.

Chehalis, Wash., population 5,630, took bids for the period May 1, 1953 to May 1, 1954 on the hauling of di

gested sludge. The award was made to a contractor who submitted a bid of $400 per year. The wet sludge is

picked up at the treatment plant and the contractor must meet all require

ments of the state health department. Approximately 142,000 gal. of sludge with a 7 per cent total solids content

and 35 to 50 per cent volatile solids were used in 1954, primarily for hay crops. The revenue received averages

approximately $10 per ton on a dry weight basis.

Issaquah, Wash., population 1,200, has an arrangement with a local farmer for hauling liquid digested sludge as it becomes digested. Ap proximately 15,000 gal. of thin sludge is disposed of every six weeks. The

farmer makes no charge for hauling nor does the city receive any revenue.

In less than one year the hay fields re

ceiving the sludge have improved crop

production from poor to excellent.

Shelton, Wash., population 5,250, has hauled liquid digested sludge in a

city-owned truck only once since the

treatment plant was placed in opera tion in 1951. Although there are no



Vol. 28, No. 3 SLUDGE DISPOSAL 325

sludge drying beds available, the di

gester has had ample storage capacity and private haulers have taken most

of the available sludge.

Air-Dried Digested Sludge

Drying of digested sludge on drying beds has been an accepted method of

dewatering sludge for many years. The problem of disposing of the dried

sludge still remains, and today the cost

is increased due to the steady rise in

labor costs.

Typical methods of disposing of air

dried sludge at three sewage treat

ment plants follow.

Vancouver, Wash., using glass-cov ered drying beds, produces a well

dried digested sludge which is ground and sold in 80 lb. sacks. The product is marketed under a trade name, and

sold for $2.50 per sack. Some bulk

sales are made and the city uses some

sludge on the parks and other mu

nicipal property. Receipts from sludge sales have been

as follows:

1953 $1,593.50 1954 920.63 1955 (7 mo.) 837.92

The cost of processing is carried as

operating overhead and is difficult to

calculate. Supervisory personnel are

of the opinion that the income pays for extra labor, sacks and other han

dling charges. Oentralia, Wash., grinds and sacks

a portion of the sludge dried on open

drying beds. Some sludge is sold in

bulk quantity and there is no charge for sludge removed directly from the

beds by those interested in hauling it

away.

Because of the relatively small quan

tity of sludge sold ($344.06 worth in

1954 and $351.00 worth in 1955), the financial return probably would not

pay for the extra labor and sacks.

City officials, however, believe the ex

cellent public relations resulting from

having sacked sludge available to the

citizens has been well worth the ex

pense.

Seattle, Wash. (Diagonal Way

Plant) disposes of its air-dried di

gested sludge to other departments of the municipality for use on public property. The sludge is removed from

the beds at no cost to the treatment

plant operating fund.

Filter Dried Sludge Raw and digested sludge both may

be dried on vacuum filters prior to

disposal. The examples cited, how

ever, filter only digested sludge. Sludge that is sold usually must be air dried or stock piled before being hauled away.

Kirkland, Wash., population 5,500, sells digested sludge on a sliding scale

of $3 to $5 per cubic yard depending on the quantity purchased. Receipts vary from $6 to $22 per ton on a

dry-weight basis. Filtering costs aver

age $6.25 per cubic yard for chemicals, labor and maintenance, but not includ

ing amortization of filter equipment. On a ton basis the cost is $24.

Enumclaw, Wash., population 2,900, sells 100 lb. sacks of air-dried, di

gested and filtered sludge for $1.50. It is estimated that income just about

pays for the cost of chemicals, ex

clusive of labor and amortization.

Tacoma, Wash., population 100,000, makes no effort to sell the filtered

sludge. Filtration costs, not including amortization, average $11.18 per ton.

Lagoon Disposal

Where cheap land is available ad

jacent to the treatment facilities the

lagooning of digested sludge is an eco nomical method. The time eventually arrives when either the sludge must be removed or new lagoons con

structed. Two municipalities report as follows on their sludge lagooning operations.

Portland, Oregon, has no sludge drying beds, consequently the digested sludge is lagooned in an area adjacent




to the treatment plant. No serious

problems have been encountered with this method since the sludge averages 33 per cent volatile solids and 10 per cent total solids.

Sufficient area is available for sev eral more years of lagooning before

more area or another disposal method will be necessary. Sludge in the la

goon measures 2 to 3 ft. in depth at

present. Several small drying beds

permit the drying of sludge for use on the plant grounds.

Tacoma, Wash., has been lagooning digested sludge in a low area adjacent to the treatment plant to save filter

operation costs. The digested sludge averages 12 per cent solids and 40 to 50 per cent volatile matter.

Lagooning has not resulted in any se

rious problems and has saved consider

able money. Following a newspaper

story on the availability and value of

sludge, the public took a considerable

quantity of dried sludge from the la

goon.

General

In the state of Washington, disposal of liquid sludge from the digester to a single contractor has been the most

successful method from a financial

standpoint. Where this method can be done with a minimum of health hazard it certainly can be encouraged. To

date, there has not been a reported record of any health problem in the state of Washington resulting from the use of digested sludge. It is true that

liquid digested sludge is theoretically dangerous since it could contain patho genic organisms.

Practically all sewage treatment

plants serving populations of fewer than 10,000 people use open drying beds and make no serious attempt to

sell sludge. Usually the dried sludge is given away or used on municipal

property. To date, sludge disposal has

not been a serious financial problem and little effort has gone into develop

ing a market. With some promotional

activity and understanding of the

agricultural needs, undoubtedly suffi cient revenues could be obtained to

make disposal financially profitable.

Legal Implications

In connection with prescribing the limitations and procedure regarding the use of sludge, the Washington State Health Department has in the

past taken the position that the return of digested sludge to the soil should be

encouraged, provided a low degree of health hazard can be maintained in the

particular method of operation. The Rules and Regulations of the

Washington State Department of

Health, Sec. 11, Part 3, Book V, reads as follows:

"Section 11. Use of Sewage Sludge for Fertilizer. The use of sewage sludge for

fertilizing material shall be in compliance with the limitations and procedures as may be prescribed by the State Director of

Health; * and the owner shall notify the

State Director of Health of an intended use of sludge as a fertilizing material."

The Washington State Attorney General's Office on June 2, 1949, ren

dered the following opinion concerning the liability of a city arising from the use for fertilizer of sludge from a city sewage treatment plant.

"The city stands in the position of manu

facturer of the sludge. Normally, a

manufacturer is not liable to persons other

than the buyer who may be injured by his

product unless he is guilty of negligence, deceit or breach of warranty. In the case

of inherently dangerous articles such as

explosives, unwholesome foods, poisons, and possibly untreated or semi-treated sew

age, a manufacturer may be liable if he

fails to give adequate warning of the na

ture of the product. "Even assuming that the sludge is classi

fied as an inherently dangerous product,

putting the highest degree of care on the

manufacturer, it is our opinion that a city would not be liable for damages resulting from the use of sludge if each contract

* See Appendix I.



Vol. 28, No. 3 SLUDGE DISPOSAL 327

of sale contained a sufficient warning of the possible dangerous nature of the sludge and the necessity of securing the approval of the Department of Health before using the sludge as a fertilizer. A clause similar

to the following would be appropriate: "

'The buyer understands that sludge pur

chased under this contract may be dan

gerously unfit for use as fertilizer and that the use of sludge for fertilizer is

subject to the approval of the State Di rector of Health according to Section

11, Part 3, Book V of the Rules and

Regulations of the State Board of Health. The seller makes no warranties

or representations as to the condition or

fitness for any purpose of the sludge.'

"Alternatively, non-liability may be se

cured by having all sludge inspected at the

plant and selling only approved sludge for fertilizer."

The State of Washington has a basic Fertilizer Act, Chapter 15.54 Revised Code of Washington (1953) adminis tered by the State Department of Agri culture, which requires registration and a fee of $25 for each brand name

and sets up definitions of various ferti

lizers, agricultural minerals and limes,

etc., and provides certain labeling re

quirements. The act would cover the

sale of sludge if it is processed in any

way, but if it is air dried and handled

like manure, the present policy has

been to consider it as such and as

manure, registration is not necessary.

Reference

1. " Utilization of Sewage Sludge as Ferti

lizer.'? Manual of Practice No. 2, Federation of Sewage Works Assns.,

Champaign, IU. (1946).

APPENDIX I.?GENERAL GUIDE FOR THE UTILIZATION AND

DISPOSAL OF SEWAGE SLUDGE. WASHINGTON STATE

DEPARTMENT OF HEALTH

It is the intent of the Washington State Department of Health in deter

mining the limitations and procedure for the utilization of sewage sludge to

encourage the beneficial use of such

sludge so that the disposal by treat

ment works will not become an un

necessary financial burden, provided, however, that a low degree of health

hazard can be obtained in the particu lar method of operation.

In evaluating and setting up spe cific limitations and procedure in any intended use of sludge, consideration

will be given to the quality of opera

tion, as reflected in the operation re

ports, design capacity and field in

spections.

Methods of Disposal

Raw Sludge

The use of raw sludge (undigested) as a fertilizing material in any form

is not permitted. Septic tank sludge would normally be considered as raw

sludge. Raw sludge may be disposed of by proper burial.

Improperly Digested Sludge

Sludge that has not been properly

digested, or a mixture of raw and di

gested sludge should be considered as

raw sludge.

Digested Sludge?General Conditions

With the exception of proper heat dried sludge, all digested sludge used as a fertilizer shall be disposed of in

such a manner that the purchaser or user has read and signed a statement that he or she understands the follow

ing, or a statement is provided with each lot of sludge which reads sub

stantially as follows:

"The buyer or user of this material understands that it is digested sewage

sludge and should not be used on root

crops, or low growing vegetables, fruits and berries, which are used for human

consumption. The seller or furnisher makes no warranties or representation




as to the condition or fitness of the

sludge for any purpose. "Persons finding it necessary to

handle this material should immedi

ately following wash their hands care

fully with soap and hot water."

Digested Sludge?Specific Conditions

1. Liquid sludge may be discharged at the site of the sewage treatment

plant; under pressure below ground in orchards, hop fields and other crop land where there is no danger of con

taminating low growing crops or ber

ries, or leaching out on to the ground surface; or on fenced hay fields pro vided cattle are excluded for a period of 45 days. The treatment plant op erator must maintain complete records on the use of the sludge. Handling of the sludge must be done in a sani

tary manner.

2. Air-dried sludge must be dried for at least 30 days or until it can

be handled with a pitchfork. Use on root crops, or low growing vegetables or fruits is permitted if the sludge is applied and plowed under six

months prior to planting. Stock-piling the sludge several months prior to use in landscaping work is recommended.

Air-dried sludge having a moisture content of less than 10 per cent may be sacked and sold to the general pub lic provided the statement given under <( Digested Sludge?General Condi

tions" appears on the bag. 3. Vacuum filtered sludge that has

been stock-piled for 30 days subse

quent to filtering, may be considered as air-dried sludge.

4. Heat-dried sludge which has been

properly dried can be used generally, without special restrictions.

CORROSION IN SEWAGE WORKS *

By Edwin R. Stowell

Senior Sanitary Engineer, California State Division of Architecture,

Sacramento, Calif.

Negligence in the control of corro

sion frequently has been due to a lack of knowledge in understanding the corrosion factor. Specific instances

may be observed where abnormal or

excessive deterioration of plant struc tures or equipment has occurred, due

primarily to corrosion. In many cases

this has taken place without the op

erating personnel recognizing that cor

rosion was active.

It is not anticipated or expected that all sewage treatment plant operators will become corrosion experts. How

ever, it is the obligation and duty of

all operating personnel to detect and

report excessive wear or deterioration

of treatment facilities intrusted to

* Presented at 1955 Annual Meeting, Cali

fornia Sewage and Industrial Wastes Assn.;

Riverside, Calif.; April 27-30, 1955.

their care, no matter what the cause of the deterioration.

In support of the premise that the role of corrosion may be overlooked in the wear of equipment, a section of

sludge scraper chain was removed from a primary clarifier. Inspection revealed that one side of the chain had been worn or corroded to a thin knife

edge section, while the other side had retained its original cross section. This rather familiar "wear" pattern of

scraper chains is considered by many operators to be due solely to mechani cal wear. The condition, however, is

largely a result of corrosion forces, possibly in combination with erosion or mechanical wear, whereby new

metal is constantly being exposed to corrosion. The important point is not that this excessive and localized loss



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Sludge Disposal Practices in the Pacific Northwest