All of these environmental considerations require sound sludge management and consideration of methods that will have the least environmental impact and be of the greatest benefit to the community. A confusing aspect of sludge man- agement is the many options available for treatment and dis- posal. Similarly, new processing techniques that have been developed and others that have matured from information available from research engineering and experience in this area confuse the matter.

WHAT IS SLUDGE? Sludge is the solids removed from wastewater during treat-

ment and concentrated for further treatment and disposal. As wastewater treatment standards have become more stringent, so has the volume of sludge increased. The further handling, treatment, disposal of sludge has become a complex manage- ment/economic/regulatory/ecological burden. The cleaning process at the wastewater treatment plant is fairly straight for-

ward. First, large solids are settled out of the wastewater. Next, the smaller solids and the dissolved materials are biologically removed from the wastewater using naturally occurring microorganisms, which feed on the contaminants in the waste- water. Finally, the excess microorganisms used to clean the wastewater is settled out. The settled solids and the excess microorganisms are called sewage sludge. (See Figure 1).

On the average, the wastewater treated from one typical person generates over four gallons per week of sewage sludge which much be disposed of. For a small city of 400,000 peo- ple, that’s over 250,000 gallons of sludge per day! The waste- water residue (sludge) is usually a 3 to 7 percent watery solution of solids removed during treatment and contains a wide range of compounds from domestic, commercial, indus- trial sources that were previously discharged into the environ- ment. Approximately 8 million dry tons (or 800 million to 1.7 billion wet tons) is produced yearly at more than 15,000 pub- licly owned treatment works (POTWs) in the United States.

28 The Nafioiinl Ei~viron~iiental Jouriial March/April 1993

GHT REDUCTION

-REDUCES FUEL REQUIREMENTS

Photo. Aeration basin at a wastewater treatment facility in L f tleton, New Hampshire, which sewes as the treatment facrhty for the town of Littleton.

MANAGEMENT AND DISPOSAL OPTIONS It is important to understand the various methods that gen-

erate sludge from wastewater treatment operations. It is equally important to identify the individual methods, their advantages and disadvantages and ultimate impact on the environment. The most common methods of sludge manage- ment today include:

Landfilling - At the wastewater treatment plant, water is removed from the sludge to concentrate it from 4 percent solids and 96 percent water to 25 percent solids and 75 per- cent water. This dewatered sludge is then brought to a landfill by truck. Lime is often added in an effort to control odors.

Land Application - At the wastewater treatment plant,

water is removed from the sludge to concentrate it from 4 per- cent solids to 25 percent solids. The sludge may at this time be mixed with wood chips and allowed to decompose. The decomposed mixture is called compost. Dewatered sludge or compost product may be used to fertilize crops, trees and can also be used for land reclamation projects. These materials can be applied as a ground cover, or can be turned into the ground during application. The 25 percent solids sludge can also be dried to 95 percent solids and pelletized, and the pel- lets can be used as a soil conditioner in potting soils or on golf courses. Land application of sludge uses the nitrates and the phosphates nutrients found in sewage sludge as fertilizer and soil conditioner.

Tlic Notional Envivonmental Joirrnol March/April 1993 29

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TABLE 1

Pretreatment Screening Strainers

0 Primary/Secondary/ Chemical Treatment

0 Gravity Settling Flotation Biological Treatment

Aerobic Digestion Anaerobic Digestion

Conditioning Inorganic Organic Thermal

0 Landfill 0 Ocean Dumping 0 IncineratiodCombustion

Landfill Dedicated Land Disposal Drying Beds HeatDry Pyrolysis

Dewatering Strainers Filter Presses Belt Filters Vacuum Filters Centrifuge Drying Lagoons Drying Beds

0 Drying Rotary Kiln Flash Dryers

Incineration/Combustion Composting

Aerated Pile Mechanical

Chemical Fixation Encapsulation Earth Worm Conversion

0 Cropland Forests Land Reclamation Raw Material Recovetv

Incineration Compost Bag (Market as Fertilizer) VerTech Aqueous Phase Oxidation

e'

Incineration - At the wastewater treatment plant, water is f removed from the sludge to concentrate it from 4percent solids to 25 percent solids. The sludge is then further dripd tn gre.lter than 80 percent solids to ensure proper burning in the incinerator. The materials are then combusted in the incinerator.

These methods of sludge management are becoming increasingly scrutinized by regulators, legislators, and the pub- lic at large. Air pollution, odors, truck traffic, water pollution, ash by-products, aesthetics and site availability are issues that greatly concern a local community. These concerns can be adequately controlled using modern techniques, but with increased costs associated with sludge management.

Wastewater treatment and wastewater solids and disposal systems must be selected to ensure the most efficient use of resources. Far too often, not all considerations are seriously made, resulting in expensive and environmentally adverse tech- nology use. Sludge disposal systems must be technically feasi- ble, reasonably cost effective, and environmentally acceptable and implementable in a required time frame.

The ultimate management goal for sewage sludge is reuse, but its name carries bad connotations for many people. Sewage sludge are biosolids, primarily organic solid products produced from the wastewater treatment process, and can be beneficially recycled or disposed. Safe and efficient methods for managing and reusing sludge or biosolids are now available due to technical breakthroughs in the field. Table 1 reviews some treatment and disposal options for sludge.

Wastewater treatment and wastewater solids and disposal

~

Circle 133 on card. 30 The National Environmental Journal March/Aprill993

systems must be selected to ensure the most efficient uses of resources such as money, materials, energy, meeting environmental regulations and being acceptable from a public as well as an environmental point of view. It follows, therefore, that in choosing a sludge disposal method, a resource-efficient and environmental- ly sound system must be chosen from the many disposal options available. Such decisions must be based on all treatment options and their relation to other critically linked factors such as sludge transportation, storage, relat-

VERTECH PELLETIZE COMPOST

ODOR POTENTIAL LOW MEDIUM HIGH

RESIDUAL TOXICITY NEGLIGIBLE VARIABLE LOW - RESIDUAL VOLUME LOW MEDIUM HIGHEST

RESIDUAL REUSE YES YES YES

TOTAL COST $/DT

DEWATERING INCLUDING 300-400 400-500 350-750

DEWATERING SLUDGE & LANDFILL GASIFY INCINERATION

HIGH MEDIUM MEDIUM

VARIABLE HIGH VARIABLE

HIGH MEDIUM LOW

NO UNKNOWN UNKNOWN

350-900 UNKNOWN 400-600

SITING CONCERNS I LOW I MEDIUM 1 MEDIUM I MEDIUM I HIGH I HIGH

Flexibility Reliability Sidestream Impacts I

ed impacts-technical, regulatory, and environmental. The proper selection of successful sludge management dis-

posal systems depends on a rigorous selection process. Sys- tem selection can be complex and a methodical approach is required, comparing all of the available options.

Sludge disposal options can be selected by: Evaluating relevant information

Narrowing the list of candidate options Identifying options /

SELECTING THE BEST SYSTEM Sludge management must meet the prime function of relia-

bility as well as satisfy the following conditions: Must be legally acceptable. Processing and disposal options must be readily available. Environmental impacts and health risks must satisfy public

Must be cost-competitive. Necessary equipment and reliability must be available. An operational system must follow almost immediately after

' implementation. Must be straightfotward with assured financing. It is important for a management system to be able to accept

all of the sludge all of the time, under all circumstances. Factors involved in selecting the best sludge management

system must be identified in any consideration of determining which is the right system to use. Potential criteria to be consid- ered include: 1. System Flexibility -ability to respond to new technology,

changes in regulations, changes in capacityAoads. 2. Reliability - probable fail rate, backup requirements,

required operator attention, vulnerability to labor. 3. Production of significant related environment prob-

lems 4. Performance data 5. Technology available 6. Financing 7. Computability -with land use, solid waste/air pollution,

existing treatment facilities. 8. Energy requirements - operation/construction/energy

recovery, credits for use of products. 9. Costs and benefits - capital costs, operating and mainte-

nance, revenues, costhenefits. 10. Public health - pathogenic organisms, toxic organics,

heavy metals, soil effects, water quality impacts such as groundwater, surface water, marine environment.

11. Effects on air quality - odors, aerosols. 12. Effects on natural resources and the environment -

depletion of resources, effects on the ecosystem, and depletion of environmental resources.

13. Safety - effects of sludge transportation and operation of a sludge management operation on the public.

as well as regulatory requirements.

Public Health Ecology Global Warming

14. Administrative burdens - level of effort, marketing, juris-

Candidate systems can be viewed from the array indicated in Table 2. Factors that affect any system's selection have been reviewed in this section. Sludge management options and the advantages and faults of the principal potential options are indentified and a rating system for evaluation of various options are shown in Table 3.

WHY BE CONCERNED WITH SLUDGE? About 8 million dry tons (800 million to 1.7 billion wet tons)

of wastewater sludge is produced each year at more than 15,000 POTWs in the United States. Approximately 97 percent of POTWs generate less than 10 dry tons per day (1 000 wet tons). The remaining 3 percent, or about 400 POTWs, gener- ate more than 10 dry tons per day and account for approxi- mately 70 percent (or 5.4 million dry tons) of sludge produced.

Organics-protein, carbohydrates, fats, oils, greases, chemi-

Pathogens and microorganisms - bacteria, viruses, parasites. Heavy and toxic metals. Toxins - pesticides, household and industrial chemicals. All of these present a risk to humans and the environment,

thereby presenting not only a sizable disposal problem, but obvious risks and hazards. Risk to man and the environment is shown in Figure 2.

As most POTWs are publicly owned or funded, the costs of sludge treatment and disposal is a significant factor affecting almost everyone's pocketbook. An important consideration in

dictional disputes, public relations.

Wastewater sludge typically contains:

cals, etc.

0 With Land Use Plans 0 With Area Wide Water I 0 Existina Treatment plant

I TABLE 3 I

I Level of Effort Marketing Responsibilities Jurisdictional Disputes

I u

Public Relations

I I 0 Financing I Several

The National Environmental Journal March/Aprill993 31 _rr__ -1- _- - _I" _-___. "_" " _ - .

e

I

to regulate use and disposal of wastewater sludges Each of the predominant sludge treat- ment methods listed above con- tributes to environmental pol- lution and wi l l become more restricted and prohibited in the foreseeable future

Another concern is that the predominant methods used today incorporate old technolo- gies that have been around for a long time and really only add to the problem in many cases Land disposal dates back to when Adam threw his first apple core away Sludge farming and using domestic wastes as fertil- izers have not been viewed effi- cient or acceptable since the latter part of the 19th century and the advent of the modern fertiliz- er industry. Incineration, proba- bly the most modern of the above technologies, dates back

costs overlook is cost-competitiveness rather than "quick fix" low cost options. Most sludge treatment methods are expen- sive and pose another demand to the financial resources of communities involved.

Wastewater treatment and subsequent sludge disposal is regulated by various laws. Section 405 of the 1987 Clean Water Act requires the U.S. Environmental Protection Agency

to man's discovery of fire. Alternates that can be cost-effective, efficient, and environ-

mentally acceptable are available today. While the technolo- gies in use today have been an important part of practice with a long history, there is a substantial amount of controversy, which continues to intensify.

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ENVIRONMENTAL IMPACTS Sludge management can have substantial

environmental impacts-for better or worse, depending on the option selected. From an overall prospective, when looking at sludge management options, effects of the following factors should be considered.

Air Impacts - particulates, odors, air cont-

truck traffic to transport end products, air impacts from the generation of raw materi- als used in the process. Water Impacts - surface water runoff,

ground water impacts due to leachate, dis- charge streams, odor problems requiring further controls. Land Impacts - soil contamination with

excess nutrients and heavy metals. Land use restrictions due to zoning. Regulatory Requirements - any sludge

management system requires a permit. Any technology has to meet federal and state laws and limits are set ljased on no effect on human health. Recent and increasingly stringent waste-

water treatment requirements have created a new solid waste problem to deal with, namely, sludge. Sludge management is neither inex- pensive nor technically simple and opens up

QminQntc frnm trnitmnnt n i v n n l l i Itinn A i i n tn UII I I I IUI I1U IIVIIa I I b U L I 1 I b I I 1 , UII p / l l U l l W l I "UG L"

new environmental concerns. 0 Reader Interest Review. Please circle the appropriate number on the Reader Service Card to indicate the level of interest in this article.

32 The National Environiiiwtnl journal March/April 1993