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Sludge digestion & disposal.

The early stages of conventional wastewater treatment are directed toward solids removal from the waste stream flow. In this sense, the term "solids" refers to those materials that readily settle in two hours or shorter, or those removed by flotation. These materials are separated from the waste stream in primary treatment - screening, grit removal, and primary settling or clarification. Secondary treatment also produces solids, or sludge, sometimes designated the "underflow." Screenings and grit are collected separately, usually because they require disposal techniques different from sludge. Flotation and settling tank scum is usually combined with the sludge for treatment.

Sludge treatment can be the plant operation's most complex process. While the volume handled is small compared with the main flow, unstable organics are present in highly concentrated form. The sludge may be stabilized by anaerobic or aerobic digestion, or in some cases, economically destroyed by incineration or similar means.

In either case, reducing high water content is important and is accomplished by thickening, and for incineration, vacuum or belt filtration, or centrifugation. Anaerobic digestion is the most widely used process, frequently preceded by thickening and followed by air drying on sand beds. Occasionally digested sludge is dewatered mechanically in filters or centrifuges.

Eventual "disposal" is ideally by some means that will use the end product beneficially. The final rules on sewage sludge use and disposal were finally established in late 1992. Issued under Part 503 of CFR 405(d) of the Clean Water Act, the regulations should help treatment operators develop sludge programs suitable for their particular facility. It will also help educate the public and make sludge end-use more acceptable. One option is land spreading the material as part of a reclamation project. Another alternative is conversion and sale as fertilizer or soil amendment. Sludge has also been used as landfill cover material. Sludge disposal by digestion, then, can be a useful by-product by reclaiming both digester gas and solid organics. Contact state regulators or the regional EPA office for more information. Coastal communities previously had the relatively inexpensive option of sea disposal with barges - this practice has now been banned.

The Water Environment Federation has suggested that the word "biosolids" be used to describe treated sludge. The idea that sludge that has been processed can be used for beneficial purposes is one that should be readily communicated to the public. The WEF defines biosolids as a "primarily organic solid product, produced by wastewater treatment processes, that can be beneficially recycled."

Waste activated sludge is sometimes considered a separate problem. It produces large quantities and is highly dilute. Aerobic digestion is frequently employed as well as lagoon disposal.

When discussing and specifying biosolids/sludge quantities and systems, care must be taken to express the units of measurement consistently. Intermixing wet tons, dry tons (short tons or metric tons?); dry weight or wet weight; percentage of total solids, and other parameters would produce confusion at the very least. This unfortunately is easy to do when different manufacturers use different parameters to rate their equipment. Government agencies and consultants may specify systems using different measurements. Quite simply, consistency in terminology is required for any given project.

ANAEROBIC DIGESTION

Tank Design & Capacity

Current general practice uses concrete for digestion tanks, either poured in place or prestressed. The shape may be cylindrical or rectilinear. Many companies supplying concrete water storage tanks can also supply concrete structures used in wastewater treatment operations.

Fixed covers are sometimes used over the tanks. They can be a spherical dome shell of concrete, prestressed at the abutment ring. The roofs can be designed to carry a super load of lean concrete or asphaltic concrete to serve as a counterweight against gas, pressure, and as insulation to reduce the temperature differential across the dome shell. Floating and structural covers are discussed below.

Provision must be made for introducing fresh sludge and withdrawing digested material. Methods for liquid removal, known as supernatant (displaced by fresh sludge), and for collecting and disposing gas, are also required.

Generally 2 to 5 cu ft per capita must be provided in heated tanks - more in unheated tanks. If ground up garbage (from home disposal units, etc.) is mixed with the sewage, the capacity must be increased.

Two-stage digestion is desirable. Among the advantages claimed is a clearer supernatant liquor from the latter tank; necessity for heating only a small primary tank, thus saving energy; and more complete digestion. With nearly 75 percent of the digestion taking place in the primary tank, most of the gas is formed there.

Biological Action

Digestion is a fermentation process in which microorganisms convert the carbon and hydrogen of organic matter to carbon dioxide and methane, leaving a stabilized residue. Where insufficient organisms are present or their biological action is inhibited, low digestion rates result. It is also obvious from digestion system studies, that microorganisms - more specifically - bacteria, are selective in their food supply and hence, an environment must be provided for optimum growth, enzyme production, and activity. Thus, if methane formers are not present, there will be little or no methane production.

Anything that hastens digestion makes possible reducing the tank's size. Aids to digestion are heat; regular frequent additions of fresh sludge; breaking up and submersion of scum; and maintaining pH between 6.9 and 7,6. In addition to seeding, other materials may be added to create a favorable gas production environment.

Additives to hasten digestion or to hydrolyze or emulsify and digest difficult materials such as oil and grease may consist of bacterial cultures specific for particular problems. These additives also include enzymes and nutrients.

It is known that bacterial "acclimatization" to a particular environment occurs over time. This could be the result of mutant strains. These so-called mutant varieties may possibly be bred to attack exotic chemical compounds (e.g. dioxin).

Chemical additives sometimes help regulate pH. Sodium bicarbonate is a buffering agent for this purpose.

Sludge Handling & Measurements

Transferring raw sludge from clarifiers to digester or from digester to sludge disposal facilities presents problems of removing too much water with the sludge. This is especially critical when transferring raw sludge to the digester. Various devices can help the operator control the transfer operation, restricting the solids content to an optimum density range.

Some digestion tanks are provided with collectors that move sludge to a central well or end hopper, similar to those used in sedimentation tanks. The bottom plows' shape and their motion rate are generally designed to keep the sludge stirred up, Sweeping it into the sump for withdrawal.

Sludge decanters are useful to aid dewatering sludge drying beds. They not only provide another supernatant outlet to the underdrains from the bed enclosure top, but also provides a readily outlet for rainwater. They are inexpensive, easily installed, and can be adjusted manually or automatically.

Liquid level in sludge digestion tanks with either floating or fixed covers may be determined and recorded by an explosion-proof pneumatic transmitter with remote receiver.

When inorganic solids in the sludge accumulate to a degree that impairs digestion efficiency the customary procedure is to shut down the digester and remove the solids. Scum and grit accumulations are removed at the same time.

High pressure pumps, hoses, nozzles, and other accessories can aid in washing the digester's various components and walls. Nozzle configurations permit directing tangential sprays, for handling a cleaning operation from a manhole.

Contract Services. It is possible to have digester and septic tanks cleaned by a contractor employing specially trained crews equipped with specialized safety, venting, flushing, and pumping equipment, as well as tank trucks and other related equipment.

Gas Collection

Gas liberated during digestion is generally collected in a roof dome, from which it is piped to a gas holder, to heating or power equipment, or to a waste gas burner. Where two-stage digestion is employed, the second tank's cover can be used as a gas holder.

The gas produced during digestion is generally 60 to 78 percent methane, which is explosive when mixed with between 7 and 14 times its volume of air. The collected gas should be handled with care, although with proper precautions the gas hazard is small.

Supernatant Disposal

The liquid displaced from the digestion tank when raw sludge is introduced, called "supernatant," is high in solids and BOD and must be treated in some way. It is returned to the raw sewage in some plants, but imposes a heavy load on any part of the plant's system to which it is returned.

One problem is finding the sludge-supernatant interface so that the liquor is withdrawn without sludge. Another problem is the anaerobic nature of the liquor. Including an aeration system will use the stabilizing ability of facultative organisms and maximize the supernatant's DO content before returning it to the process.

Mixing & Scum Breaking

Scum breaking features in some round tanks include stationary and rotating scum fingers and mechanical circulating mixers for scum control on both fixed and floating roof digesters.

Algae removal is an often demanding, labor-intensive task that is the bane of the maintenance staff. Scrubbing away algae in an operating facility is dangerous at best, and it may not be possible to reach some surfaces. Chemicals have been used successfully to control this problem, but have some of their own associated handling problems and may negatively affect the effluent's discharge requirements.

Floating & Structural Covers

The floating cover, which rises and falls with the liquid contents, prevents air from collecting on top of the scum and helps insulate the digester against heat loss. It is also claimed that a floating cover reduces the amount of overflow liquor, gives constant gas pressure, and facilitates the process of two-stage digestion.

The Imhoff tank, an early design that combined clarification and digestion in one structure, lead a number of manufacturers to utilize this concept in equipment design, but avoiding the objectionable features of the intimate contact between clarified wastewater, supernatant liquor, and sludge.

Heating Sludge

A temperature above 70 [degrees] F - preferably 80 [degrees] to 100 [degrees] F - is necessary for rapid digestion. Experiments indicate that while 120 days are required for thorough digestion at 55 [degrees] F and 42 days at 68 [degrees], this period can be reduced to 30 days if the temperature is at 82 [degrees]. In other words, the tank can then be reduced more than 50 percent in size if the temperature for winter operation is maintained at 80 [degrees] F.

One method of heating tanks is by passing hot water through pipes inside the tank, as coils around the periphery, or preferably as removable banks. The water for heating tanks is generally heated by burning the gas collected from the tank, or by the exhaust from engines using such gas.

The water used in a waste heat recovery system should be non-corrosive and non-scale forming. An expansion tank or tanks should be provided at a high point in the system, which will level out surges and compensate for changes in volume, and most important, release air from the system. If the system is of any magnitude, automatic air-release valves should be used liberally. Water circulating pumps must be specified for the thermal range anticipated. Thermally controlled valves that open and close at preset temperatures may be used in the hot water system to regulate flow to heat-consuming devices.

Submerged gas burners have been used in digestion tanks. The equipment for this consists of a carburetor and a gas burner of either the open or enclosed type. The carburetor mixes air and digester gas in the desired proportion and pumps it through a pipe to the burners. In the open type burner the flame and burnt gases are discharged into the sludge. There is no heat loss, and circulation of the sludge results; but the gas mixture must be discharged under sufficient pressure to overcome the hydrostatic head.

External heaters can transfer to the sludge more heat per unit of exchanger surface than can internal pipes; are more accessible for cleaning and repairing, and avoid the temperature drops in a digester caused by adding unheated raw sludge to it.

AEROBIC DIGESTION

Intensive aeration of fresh sludge over an extended period of time can result in some thickening (two to three percent solids) and mineralization of the organic matter by oxidizing volatile solids. While, for separate digestion of solids, the detention time and tank capacity are about the same as for anaerobic digestion, the tank need not be covered nor heated in most temperate climates. Furthermore, it is claimed by adherents that final disposal of liquid and solid fractions pose few problems.

By directing the overflow from the digester to a settling compartment, the liquid fraction can be automatically decanted as it is displaced with influent digested sludge. If this supernatant requires further treatment, it may be returned to the plant influent. The settled sludge can be recycled for further thickening, dewatered on sand beds, or placed in lagoons. It is claimed that aerobically digested sludge drains more rapidly than anaerobically digested sludge and sand bed area for air drying need not be as great. Companies supplying equipment for this process may offer certain modifications and enhancements to improve performance, efficiency, or to handle specific types of flow.

GAS UTILIZATION

Gas formed in digestion tanks has a Btu value of about 600 or 700, and can be used as a source of heat by burning it in gas boilers, or as a source of power by using it in gas engines. In fact, it can be used in any way that "city gas" is used. Most plants now use it for heating water for raising the temperature in the digestion tanks, and many for operating gas engines that drive blowers for activated sludge plants, electric generators, and other equipment. Where it is used in gas engines, the exhaust from these is employed to heat the water for the tank heating system. In several cases, current generated by means of this gas is supplied for operating sewage pumps, and in a few for pumping done by the water department, for street lighting, and other municipal uses.

The amount of gas produced from a digestion tank can vary from one to three or more cu ft per day per capita. Where garbage is digested with the sewage, the amount of gas per capita may double this amount where all the city's garbage is so handled. Some types of industrial wastes also increase the amount of gas produced by the sewage.

In general, about 9.5 cu ft of gas is obtained per pound of volatile matter added to the digester; or 3.8 to 5.7 cu ft per pound of solids in the raw sewage removed by primary treatment. Using gas in some way should be part of the design.

[H.sub.2]S in Gas

Gas obtained from the digesters, although principally methane and carbon dioxide, contains hydrogen sulfide. This gas is generally present in concentrations which, though small, are sufficient to corrode metals and damage other materials.

Among the effects are pitting of the gas engines and clogging of piston rings. The [H.sub.2]S may be removed by passing the gas through iron oxide, supported by shavings, peat, or other materials that have a similar physical structure. Another method of removing [H.sub.2]S is to "scrub" the gas with a liquid containing reacting chemicals.

Gas Storage

Pressure vessels are used for gas storage to adjust the differences in the varying rates of gas production and consumption and maintaining uniform pressure. Several plants use spherical tanks, 40 ft or more in diameter, which are made to withstand pressures of 30 to 40 lb.

Digester Instrumentation

Metering digester gas is good business practice if the gas is utilized in plant operation. Gas analyzers and combustible gas detectors are among the important safety items to found in the modern treatment plant.

Employees working around digesters should be educated in the dangers involved in smoking or the using sparking tools. The toxic nature of digester gases should also be pointed out. Safety training programs should be part of treatment plant's operational philosophy.

Gas Safety Devices

Standard equipment for any digester should include flame traps, gas pressure regulators, waste gas burners, accumulators, and drip traps.

Entrained water in digester gas can cause trouble in meters, valves, compressors, engines, and other equipment for gas utilization. Water accumulation is increased upon cooling and compressing. The entrained water may be removed by a gas scrubber.

Pipe galleries and digester control rooms require mechanical ventilation for personnel safety and protection against corrosion. Fans used for this purpose should be equipped with explosion-proof motors. Emergency and first aid equipment should be available and the personnel trained in their use.

Gas Heated Boilers

A boiler using sewage gas should be especially designed for burning such gas, since it has a different heat value from commercial gas. The hot water supply should be regulated by a thermostat, to keep the water in the tank-heating coils below a temperature that would cause deposits of dry sludge on the pipes.

Though sludge gas normally has a fuel value somewhat less than manufactured or natural gas, it is readily convertible to power by the installation of commercially available spark-ignition types of gas engines or dual-fuel engines that can operate on gas or diesel oil. The primary differences in construction between engines operating on gasoline, natural or manufactured gas, and those using sewage gas are the use of a different compression ratio and carburetor equipment. The most efficient engine operation is obtained with the highest compression ratio below the point of detonation, and the higher the Btu of the gas, the higher can be the compression ratio. This should be provided for in an engine designed for using sewage gas.

The hydrogen sulfide content of sludge gas deserves consideration in making the installation. The maximum safe concentration of [H.sub.2]S in gas for engine operation has been given variously as 5 to 50 grains per 100 cu ft of gas. Unless it is known by frequent analyses that this range is seldom or never exceeded, equipment should be installed for its control.

No harm is done by the hydrogen sulfide if no moisture is present to form sulfuric acid. Condensation of water vapor can be prevented by maintaining the gas in the cylinders at a uniform high temperature.

Where gas is used to generate electricity, efficiency seldom exceeds 83 percent. Better efficiency is obtained by directly connecting the engine to the pump or blower. In many plants it is possible to direct-drive the large unit, and also to generate current for driving the smaller traits.

Where the available volume of gas fluctuates widely, the engine is often fitted with a induction generators - essentially a squirrel-cage motor operating slightly above synchronous speed. They must be in parallel with an outside source of power. Another type is the conventional alternating current generator and exciter.

Engine Selection

The decision to convert gas to power and choosing the type and engine capacity to be employed is entirely an economic consideration. The amount of gas available is the primary criterion. Uses to which the power can be put at appreciable savings is another factor, but conversion of sludge gas to standby power can fulfill a major need in most plants.

The power available in the sewage gas from a given plant depends upon the pounds per day of volatile solids removed from the sewage treated. Assuming that 60 percent of the solids in the raw sludge is destroyed by digestion, with the production of 9.5 cu ft of gas per lb of volatile matter so destroyed; and assuming that primary treatment removes 40 percent of the suspended solids from weak sewage and 60 percent from strong, the average power available with primary treatment only will vary from 5.6 to 29.4 brake horsepower (bhp) per mgd. If activated sludge is added, this is increased to 12.6 to 44.1 bhp. If the gas engines drive electric generators, this is reduced by 25 percent or more.

The type of engine operating on sludge gas is the high-compression, dual-fuel diesel, which can operate over the entire range from minimum ignition oil requirements (pilot oil) and gas, to 100 percent oil with no gas; and the high-compression gas diesel, which operates at all times with only the pilot oil the balance of the fuel required.

Supercharged engines, with automatic throttling of air supply to maintain more constant air-gas ratio, appear the engine of choice under conditions of partial load, frequently found in engine generator sets. A further advantage of the supercharged engine is that the power output from an engine of the same number of cylinders, bore, stroke, and rpm can be increased, reducing space needs.

Dual-Fuel Engines

Sewage plant designers have found that dual-fuel engines provide a degree of versatility in that such engines may operate on oil or oil-gas combinations when sufficient quantities of gas are not produced to operate the engines as needed.

Accessories

It is essential to be able to start up an engine and put it on the line quickly. Compressed air for the diesel engines or a battery operated starter may be used. The starting air compressor may be equipped with a combination electric motor-gasoline engine drive. It is important to keep all engines "warmed up," at a temperature of about 160 [degrees] F. This can be done by circulating engine cooling water through idle engines, or using an electric immersion heater in the engine cooling water system, controlled by an aquastat. These heaters are sized according to the cubic inch displacement of the engine.

Complaints are always possible due to noise or vibration. Adequate silencers must be provided, of such size as not to create excess back-pressure. Foundations should be carefully investigated and adequately designed; the engine and foundation are often isolated from the engine room slab. Engine back-pressure is very critical and can be checked by placing a manometer on a tap made on engine exhaust, ahead of heat exchangers, boiler, stack, or any device capable of creating back pressure.

CHEMICAL STABILIZATION

Various methods have been developed over the years to stabilize sludge by using chemical means, which is a continuation of the physical-chemical treatment process. Some of these processes work at ambient temperatures and at low pressures utilizing readily available chemicals such as chlorine. The solids can then be dewatered on sand beds or by a vacuum filter. The supernatant is then returned to the primary clarifier.

DEWATERING SLUDGE

Air Drying & Removal

In air drying, the sludge is drawn onto an underdrain bed of sand, crushed anthracite coal or similar coarse media, or specially made draining screens. Part of the water drains through the bed, while the large surface exposed permits gradual evaporation of part of the remainder, producing a cake easily turned with a spade with about 60 percent moisture.

In climates where there is little rain and a high evaporation rate, sludge dries in a few days. Criteria for design vary with the type of treatment process producing the sludge and the area of the country. The range is 0.5 to 2.0 sq ft per capita. Frequently, 25 percent less area is required when a bed is covered.

A well-digested sludge dries readily without nuisance. Gases are formed that tend to float the mass and release water from it. Gasification can be increased by adding alum to the sludge as it flows on the beds in the proportion of 1 lb to 100 gallons. Polymer flocculent conditioners can hasten dewatering. Lime is helpful in the event odors or fly breeding develops.

Greenhouse types of glass enclosures over sludge drying beds are employed at many plants. They serve a multiple function, principally preventing rain and snow from interrupting the air drying process. They also provide odor control and present a pleasing appearance in distinguishing the existence of the sludge bed. In areas where there is much rain or freezing, they may reduce the drying period by 50 percent or more during unfavorable months. Adding automatic ventilation control can further accelerate the drying process, thus reducing the total area required for the beds.

The bottom chord of the roof truss in the enclosure should be high enough above the sand level to allow sufficient working room. If monorails are used for removing the sludge, minimum distances should be carefully checked to allow sufficient room to facilitate unloading the buckets onto trucks.

Standard designs have been developed by manufacturers, consulting engineers, and state health boards. Manufacturers offer a number of variations employing standard widths, length of truss spacings, eave heights, etc., so probably the best plan in asking for bids is to specify the minimum acceptable area together with approximate dimensions desired, permitting the manufacturer to offer the best design to meet the general requirement.

Fiberglass and aluminum framing for glass enclosures are often used for enclosing the beds.

Removing Sludge. For removing the dried sludge from the beds, a number of mechanical aids have been developed. Use of buckets carried by monorail supported above the beds has been most common practice, especially with enclosed beds. Monorail equipment for either hand or motor operation may be obtained from greenhouse manufacturers and other companies. Some plants use track-type tractors with bins slung on each side, like saddle-bags. Others use a front-end loader on a small tractor, with bucket capacities available in almost any size. In some large plants, narrow gauge tracks are laid between the beds to permit using dump cars. Another method is removal by a conveyor belt, onto which the sludge is shoveled and by which it is carried to and beyond the end of the bed.

For those smaller treatment plants that only have periodic needs to dewater solids, some companies have mobile dewatering equipment and provide related services for sludge handling.

Vacuum Filters

The usual vacuum filter design involves a drum suspended in a vat containing the sludge to be dewatered, about 25 percent of the drum circumference being submerged. Radial partitions divide the filter drum into compartments, each subjected to suction and to pressure alternately during each revolution. The drum surface is covered with a filter medium through which the water is drawn as the drum revolves through the sludge in the vat. The solids are deposited on the surface of the drum in a layer or cake 1/4 in. or more in thickness. As this layer rises above the vat it is further dewatered by drawing air through it. Just before the cake reaches the vat again it is removed from the drum.

The performance of a vacuum filter is measured by the pounds of dry solids in the sludge cake produced per square foot of filter surface per hour, which varies from one to eight, but is usually four to five, with cake moisture of 60 to 80 percent. The sludge is usually conditioned by chemicals such as ferric chloride and lime or polymers before being applied to the filter. The capacity of a filter for a given sludge depends on the chemical feed accuracy.

The complete installation consists of sludge pump or bucket elevator, chemical feeders, sludge conditioning tanks, vacuum filter, vacuum receiving tanks, vacuum pump, filtrate pump, filter cake conveyor, and sludge cake hopper. These can be grouped into a small space, making a compact sludge treatment unit.

Media Types. The principles used in the vacuum filters being marketed today vary little among manufacturers, the primary difference being the kind of filtering media employed. The majority of manufacturers utilize woven fabric filter media of natural or synthetic materials.

In transferring wet sludge to vacuum filters or other dewatering devices, glass-lined pipe is useful. Any grease present will not adhere to the glass surface and clogging from this source is avoided.

Filter Fabrics. The fabrics used for filter media may be of synthetic fibers or natural. Of the latter, woolen fabric is used for acid sludge and cotton for alkaline. Nylon, Vinyon N, Orlon, Dynel, Dacron, and other chemical resistant synthetic cloths are available.

Conditioning Sludge. Chemicals are added to sludge before vacuum filtration to facilitate separation of solids and the liquid. The conditioning chemicals commonly used are ferric chloride and lime, or ferric chloride alone, depending on the characteristics of the sludge to be dewatered. Ferrous sulfate and polymers have also been used. Fresh, settled, and chemically precipitated sludges are best conditioned with lime and ferric chloride.

The amounts of conditioner used average about two percent ferric chloride and ten percent lime, on a dry solids basis, for plain settled or chemically precipitated sludge; four percent ferric chloride and eight percent lime for digested sludge; two to three percent ferric chloride for elutriated digested solids, and four percent of ferric chloride only for activated sludge.

Elutriation. Certain decomposition products held in solution in sludge when combined with the ferric salts used in pre-filtering conditioning form a gelatinous precipitate difficult to separate by any known process of filtration. If the sludge has been rapidly and thoroughly mixed with fresh water, or with the decanted supernatant liquid from a sedimentation tank the solids will be more easily separated from the water holding them in suspension.

This washing is called "elutriation," and is claimed to be capable of reducing the conditioning chemicals used by 65 percent or more, minimizing the effect of insufficient digestion, lengthening the life of filter cloths, and giving a sludge of more uniform solids content.

Weighing Sludge. Wastewater treatment plant records should include the weight of sludge cake produced periodically, and this is most conveniently obtained by weighing it while it is on the conveyor belt. Devices for weighing continuously while the belt is in motion include gauges furnished by instrument manufacturers.

Some manufacturers of conveyors and of industrial scales provide equipment that incorporates both the scale and conveyor.

Other Dewatering Devices

Sludge may also be dewatered by passage over a porous surface with agitation provided by mechanical vibration, the vibration apparently changing the physical structure. The standard plate and frame press used for decades in the chemical industry has been adapted to dewatering of wastewater sludge. Among the adaptations are the use of air compression diaphragms in conjunction with the plates and mechanization for continuous operation.

Another industrial filtration tool, the belt filter press is becoming popular in sludge dewatering applications. It lends itself to continuous automated operation including the conditioning step. It features sludge feed with conditioning chemicals to a continuous belt, subjected to the action of pressure rollers or a drum to effect dewatering.

Similarly, centrifugals have been adapted to wastewater treatment needs and are now widely used for sludge thickening and dewatering.

Thickening of sludge before centrifuging or filtration improves the dewatering capability of such devices.

SLUDGE AS FERTILIZER

Ever since Imhoff tanks were built in the 1920s, cities have pondered the possibility of making agricultural use of the sludge - biosolids - resulting from anaerobic digestion or other wastewater treatment processes. Air-dried sludge is frequently ground in a hammer mill and bagged for local sale; it is used for fertilizing publicly owned land; and it is sometimes given away to individuals willing to haul it. More recently some municipalities are marketing and selling composted sludge, helping to recoup some of the costs of treatment operations.

Larger cities developed large-scale fertilizer manufacturing plants and some are still in existence with satisfactory markets for the product. Usually, the essential processing equipment items for such procedures are dewatering mechanisms and flash or spray dryers.

Liquid Sludge. Liquid digested sludge is suitable for direct land application under situations suitable for the purpose, such as a municipality contracting with a farmer plagued with poor soil conditions. However, even when soil conditions are good, crop production can be enhanced by these additional nutrients.

One means of landspreading is to haul liquid sludge in a tank truck equipped with specially designed spreaders, auger beaters, and/or special application apparatus. In many cases the equipment can be custom built for specific application, e.g., to consulting engineering specifications.

Manufacturers of wastewater spray irrigation equipment can furnish devices suitable for handling liquid sludge.

Dewatered Sludge/Biosolids. Air-dried sludge is usually left on the beds until it can be moved with a spade, and is then carried to dumps, used for fertilizer, pulverized or shredded, or incinerated with or without further drying. Land spreading of dewatered sludge is usually handled by special surface spreading equipment mounted on a truck chassis. Two types of spreading equipment are available - the conventional rear discharge or "box" spreaders, and the side discharge or "slinger" type spreaders.

Conventional spreaders utilize chain/slat and rear beater assemblies that move sludge to the rear and distribute material behind the truck. The side discharge method uses a "V" shaped box with augers moving the material forward to a high-speed expeller that "slings" the material up to 50 feet to the side. The side discharge spreaders are nearly liquid-tight; they can handle a wide range of materials without any leakage or bridging.

A recent survey by the EPA indicates that thousands of municipalities are actively using land-spreading as a sludge disposal method routinely. Many other communities or agencies are considered one of these methods as well. Some have been instituting the practice for several decades. At one time it was permitted to include leasing land for wastewater disposal and including the cost of the lease in the proposal for funds. Municipalities thinking of going this route should check with their state regulatory agency or regional EPA office to determine availability of funds and eligibility.

Several cities have resorted to the manufacture and sale of fertilizer from wastewater sludge. Several have been conducting such operations for a few decades. Among them are Houston, Texas; Milwaukee, Wisconsin; and Winston-Salem, North Carolina.

Shredding, Composting & Solidification

Shredding. If sludge is to be bagged or hauled in dry form, it may be economical to shred it. Hammermill type shredders, cage mills, or similar grinders are suitable or can be adapted for such an operation. Some firms provide all of the equipment necessary including conveyors.

Composting. Several cities have made attempts to convert sludge to compost. This is a method of aerobic digestion. One operation in preparing compost is mixing bulking materials, like wood chips and other forms of cellulose, and windrowing the mixture for drying and aerobic digestion.

Sludge can be mixed with bulking agents with a reel type mixer in addition to auger types. Traditional auger type mixers utilize 3 or 4 heavy duty augers that force the materials together to mix and blend. The reel type uses a large diameter reel assembly and two small augers stacked vertically on one side.

The reel lifts the materials into the side augers rather than forcing the materials together, resulting in less compression, which allows air space to enhance composting and savings on reusable bulking agents. Reel type mixers are available in tow, truck, stationary, and diesel powered models.

The U.S. Dept. of Agriculture has experimented with composting dewatered filter cake from Washington's Blue Plains plant on a farm at Beltsville, Maryland. Information on research results is available from the Biological Waste Management Laboratory, ARS, USDA, Beltsville, Maryland 20705.

It has been found, for example, that the compost has a fertilizer rating of 0.9 percent nitrogen, 2.3 percent phosphoric acid, and 0.2 percent potash - low compared with chemical fertilizers. However, it is a good soil conditioner, good source of metals and a method of disposal with minimum risk of spreading pathogens. Both raw or undigested primary and secondary sludge have been composted. Economics have to be worked out locally, but should prove beneficial to a city if a continual user can be found. In general, the USDA considers the method to be technically efficient, economically sound, and beneficial.

Codisposal with Solid Wastes

Particularly in the coastal regions, where ocean dumping of sludge and refuse was finally banned after several extensions, processes that handle refuse and wastewater sludge problems in a single operation are being used. The sorted refuse, mixed with digested sludge is easily incinerated or composted.

Pyrolysis is another choice that makes possible the recovery of carbonized products and other usable materials. It utilizes a "starved air" combustion system and has been researched by a number of firms.

FILTER CAKE DRYING

The moisture content of sludge can be reduced by vacuum filters to 70 to 75 percent, or somewhat below, but this is not low enough for commercial fertilizer. Further drying is obtained by the use of heat to convert the sludge into a soil conditioner suitable for use in municipal parks or for bagging for sale.

Flash or Spray Drying

This process derived its name from the fact that the sludge particles are dried in suspension in a stream of hot gases to provide almost instantaneous removal of excess moisture.

In the more sophisticated systems, wet filter cake or suitably dewatered sludge is blended with previously dried sludge, disintegrated in a cage mill, or similar device where it is contacted with the hot gases. Gas borne sludge particles can be separated in a cyclone and returned to the feed. However, some arrangements (for spray drying) utilize liquid sludge feed and one mixes sludge with oil with subsequent oil-sludge particle separation occurring in a centrifuge.

Rotary Dryers & Indirect Heating

Rotary dryers have been used in several plants, the heat being obtained from coal, oil, gas, or municipal rubbish, or from the burning of the dried sludge. The choice of fuel depends largely upon relative costs.

A typical arrangement includes a pug mill, rotary kiln dryer, dry cyclone, web-scrubber, deodorizing afterburner, sizing screen, storage bin, and bagging equipment, if desired. Without the afterburner, normal heat efficiency is about 72 percent. With the afterburner, heat may be recovered and efficiency increased by heat exchangers in the afterburner chamber. Control of fines to assure a product of uniform quality is a required feature of any system.

Various systems of heat exchange may be used in the main drying operation, such as recirculation of hot oil, in the case of oil-fired units.

Other Thermal Processes

By the use of heat exchangers with a medium temperature range, 300 to 400 [degrees] F, and in some cases under pressure, the water retention properties are destroyed, resulting in a conditioned sludge that is easily dried. Thermally conditioned sludge cake can be land-applied, or fed to a furnace where it will combust autothermally, producing recoverable steam.

SLUDGE DISPOSAL

The concentrations of carbon and hydrogen in a material determine its heat value, and since the heat value of hydrogen is more than four times that of carbon, the relative proportion of these constituents is likewise a governing factor. Industrial wastes are primarily responsible for variations of these components and the heat value of municipal sewage sludge. In general, on a moisture-free basis, digested sludge is 59.6 percent combustible and has a heat value of 5,290 Btu per lb. For grease and scum, these figures are 88.5 percent and 16,750 Btu, respectively; for the fine screenings, 86.4 percent and 8,990 Btu; and for grit 33.2 percent and 4,000 Btu. The incineration of all these materials is, therefore, entirely feasible. In fact, if the temperature in the combustion zone is 1,200 [degrees] F or above, these materials will burn without producing noxious odors.

Incineration or other means of total destruction of sludge are particularly applicable in congested areas where land is not available for other disposal techniques. However, air pollution control must be an environmental and economic consideration - provisions of the Clean Air Act must be strictly followed.

Incineration

Incinerating sewage sludge that has been partly dewatered (as by a vacuum filter) is possible in a multiple hearth furnace. Typically, this consists of a refractory lined, steel jacketed circular unit, which contains four or more hearths, one above the other, according to the capacity required for a specific installation. In a 6-hearth furnace the top and possibly other hearths may have circular openings in the center, and intermediate ones having openings around the periphery. The sludge is pushed through the openings to the hearth below by rotating plows or rabble arms connected to a hollow air-cooled vertical central motor-driven shaft. The teeth attached to each rabble arm break up the sludge thoroughly and continuously, this materially expediting drying and combustion. The sludge is discharged from the bottom hearth as ash. In some, the hearths are divided into drying zones, combustion zones, and incineration zones; or the furnace may be combined with a flash dryer so wet raw sludge can be destroyed.

Ashes from the incinerator are collected in an ash hopper for subsequent removal. When there is a fill area adjacent to the incinerator plant the ashes can be handled hydraulically by pumping automatically from the ash hopper directly to the fill area without any intermediate handling. When such area is not available, however, the ashes can be conveyed in air suspension in an enclosed vacuum type conveyor from the ash hopper to an elevated storage bin. The dry ashes are then gravity discharged to a motor-operated rotary ash conditioner in which the ashes are thoroughly and uniformly mixed with water to a consistency suitable to suppress the dust and to allow the dustless material to be discharged to auto trucks or railroad cars for ultimate disposal off the premises.

To avoid air pollution, incinerator and dryer effluent gases should be subjected to "scrubbing" prior to discharge to the atmosphere.

Wet-Air Oxidation

The wet air oxidation method is based on the discovery that any substance capable of being burned - including human waste and organic chemicals - can be oxidized in water without flame or smoke. In the process, an aqueous solution or suspension of waste products and compressed air are pumped into a pressurized reactor and heated to from 350 to 600E At the higher temperatures, oxidation of up to 99+ percent can be accomplished leaving behind only an inorganic residue, carbon dioxide, and water. At the lower temperatures the process produces a drainable, sterile nonputrescible residue, which can be used as a soil conditioner, as landfill material, or can be incinerated, which greatly reduces the amount landfilled.

Application of wet air oxidation for the regeneration of powdered activated carbon can be done quite efficiently. Wet air regeneration allows handling of the powdered carbon as a slurry and thus, makes practical the use of low cost powdered carbon in a variety of secondary and advanced wastewater treatment flow sheets.

Fluidized Beds

In fluidized bed incinerators, a bed of sand is placed in motion by passing hot air through the medium. The sewage sludge, after being dewatered, is directed over the bed and is converted to ash. The accumulated ash is then collected in a scrubber and dewatered in an ash classifier.

An air preheater, fluidized bed reactor, and a cyclone or wet venturi scrubber are usually employed. The dewatered cake is pumped to the top of a reactor that contains the sand or other material, providing a heat sink for combustion of organic matter. Gases resulting from combustion pass through the cyclone or scrubber for removal of particulate matter to prevent air pollution when they are exhausted to the atmosphere. If heat is to be recovered, the gases can be passed through a waste heat boiler for generation of steam. Grease can be collected from plant operations and burned in the reactor. Preheating is accomplished by natural gas or fuel oil. The sludge may also enter the side of the reactor in the freeboard area. The fluidized bed operates at temperatures of 1,400 [degrees] to 2,000 [degrees] F. Terminal conditions for a waste heat boiler are full operating inlet temperature and about 450 [degrees] F at the outlet.

If heat recovery is in the form of hot process water, the waste heat boiler is not necessary and the venturi scrubber is used; otherwise it is replaced by a cyclone. The fuel is used for start-up only. When ignition temperature is attained, there is sufficient heat value in the sludge and grease to sustain combustion.

RELATED ARTICLE: Educational Exhibits Tell the Wastewater Story

Cities and counties across America face the citizens' lack of appreciation for fresh water. There is the misconception of the never-ending supply, and the lack of understanding of where water goes once it's down the drain. These problems are both costly and environmentally damaging and can return to haunt us.

In Portland, Oregon (population 503,000 with over one million people in the metropolitan area), the problems are similar. Recently Portland's Bureau of Environmental Services (BES) invested in a pro-active solution to this problem.

Seventy-five percent of the metropolitan area's sewage winds up at the Columbia Boulevard Wastewater Treatment Plant. The ongoing challenge has been educating the public about what takes place there. The city envisioned a very different type of on-site exhibit that would increase public awareness.

The city chose Portland-based Formations Inc., a master-planning, design, fabrication, and installation firm with a national reputation for engaging public interest and audience involvement and participation.

"We assimilate the core values and mission of an organization," explains Craig Kerger, Formations president and design director, "and then translate them into key-themes and take-home messages through one of the most effective vehicles to learning we've found - interactive involvement and participation."

The 1,700-sq ft, $200,000 educational exhibit opened in August 1997. Three floors of exhibit space now define the role, relations, and mission of Portland's Columbia Boulevard Plant to the city's rate payers.

On the first floor, visitors explore broad, introductory concepts focusing on the earth's water supply. They are invited to investigate the Portland watershed and the city's water distribution and sewage systems. They become aware of Portland's commitment to clean water and learn how the city fulfills that mission through its treatment plant.

Exhibits on the second floor draw visitors into the inner workings of the plant. Fascinating views of plant operations, as well as clear, engaging explanations of the treatment process, show visitors how sewage is transformed into water that's safe enough to be released into the nearby Columbia River. Historic photos are also presented on this floor. These interesting "pipelines to the past" give visitors a sense of how Portland's methods of dealing with wastewater have changed over time.

Third-floor exhibits offer views and vistas. Visitors are treated to an up-close view of headworks equipment in action, while an accompanying display catches the eye with samples of debris extracted from sewage. An elegant gallery of photographs and quotes invites the public to explore a diversity of values and priorities as different Portlanders share stories about wastewater treatment and its impact on themselves and the environment. From an observation deck, the public can enjoy a bird's-eye view of the grounds below.

Visitors can see that despite the region's diversity, a common bond unites the metropolitan area - the need for clean water. Adds Kerger, "Our aim is to reach people - to get them to pay attention and to come away with meaning attached to something they've overlooked or taken for granted in the past. We're quite pleased with the results."

RELATED ARTICLE: EPA Honors Biosolids Recycling Program

The EPA has awarded Duckett Creek Sanitary District in St. Charles County, Missouri, a "Wastewater Management Excellence Award for Beneficial Use of Biosolids." The award recognizes the District's highly successful biosolids recycling program. EPA Region 7 also awarded first-place honors for the District's biosolids recycling program in a separate judging process.

"This is a significant accomplishment for Duckett Creek Sanitary District since they were competing against much larger facilities," said John Walker, national manager of the EPA awards program. Until very recently, the District had been processing less than 5 mgd. When it surpassed that mark in 1996, it was required to compete against much larger cities such as Houston, Toledo, and Washington, D.C.

The District manages a sustainable biosolids recycling program, unlike many other wastewater treatment facilities that bum sludge or truck it to a landfill. Recycled biosolids are thoroughly tested and enhance soil quality by adding nutrients, trace minerals, and organic matter to the soil.

The District produces about 1,200 dry tons of biosolids a year, which are recycled on farm fields in the Missouri River bottoms. Most of these fields in the Greens Bottoms Levee District were heavily damaged by sand deposits during the Great Flood of 1993. Local farmers were somewhat skeptical of biosolids recycling in the beginning so District staff worked one-on-one to develop cooperative relationships and ensure good results in the field. Farmers participating in the program report that biosolids application accelerated soil recovery after the flood by several years and has increased corn, soybean, and wheat yields.

Local farmers' acceptance of biosolids application has been so enthusiastic that the District now keeps a waiting list for the program. Although the District currently produces only enough biosolids to treat 300 to 400 acres a year, local farmers have offered 1,400 acres of land for application.

Duckett Creek staff have made numerous innovations in the biosolids recycling program to conserve both energy and staff time, reducing the cost to process biosolids from $170 a ton when the program was initiated in 1989 to $28 a ton in 1996. The District also has trained neighboring wastewater districts on effective biosolids management and land application.
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Title Annotation:includes related articles
Publication:Public Works
Date:Apr 15, 1998
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