5. Sludge treatment

5. SLUDGE TREATMENT

5.1. MAIN SLUDGE TREATMENT PROCESSES

The choice of a sludge handling process is critical, as the continuity and the quality of water treatment depend on it. A municipal wastewater treatment plant produces approximately 2 litres of liquid sludge per population equivalent and per day.

Process selection is difficult and the following factors must be taken into account: - wastewater characteristics and type of treatment, - plant size, - capital resources, - ultimate use of sludge and therefore the form of the finished product.

Table 88 compares the main features of various sludge handling processes.

5.2. PROCESS DESIGN CONSIDERATIONS

To arrive at an end product whose characteristics comply with the stated yeuse objectives, a series of processes of varying complexity must be implemented in an optimum manner.

Prime factors to consider are: - storage conditions, - variable efficiency of the various unit processes.

It is highly inadvisable to store fresh liquid sludge. Fermentation makes the sludge more difficult to filter, and the risk of unpleasant odours can only increase. Treatment of fresh sludge must therefore follow as closely as possible on the heels of wastewater treatment. Thickeners should be viewed as just what they are, not as storage vessels for thickened sludge.

Sludge should be dewatered as soon as possible unless an anaerobic digestion stage is provided.

Although operating results can be analyzed in terms of mean characteristics, it is imperative to evaluate process performance with respect to less favourable values - that is the only viable means of assessing process reliability.

The engineer must always bear in mind that the characteristics of sludge from municipal wastewater are neither homogeneous nor constant. . Pipes

Pipe layout must be as rectilinear as possible. Gravity flow with low hydraulic loadings should be avoided. Where gravity flow is unavoidable, provisions for cleaning or flushing must be made, especially for fresh sludge. Head losses must be calculated with a high tolerance, with rheological rules being applied for nonNewtonian fluids in difficult cases.

Chap. 24: Municipal wastewater treatment

Table 88. Main sludge treatment processes.

Sludge type, treatment Range of use in process and dry solids Advantages Disadvantages Possible destination Europe (population)

content FRESH SLUDGE Limited capital costs High risk of odour Fresh sludge with dewa- Moderate capital cost. Highly fermentable prod- Landfill if acceptable dry- 20,000 to 50,000 tering by belt filter or cen- Small space requirement. uct. Requires lime treat- ness. trifuge. DS = 20-25% ment before and after. "Quick" solution. High quantity of sludge. High landfill costs. Fresh sludge with dewa- Average capital cost. High reagent cost. Mon- Landfill. Incineration pos- >50,000 tering by filter press. DS Good cake dryness. itoring critical. Significant sible following further = 35-40%. drying costs. drying. Fresh sludge with dewa- Product suitable for reuse. High capital cost can be Commercial-grade com- > 100,000 tering by belt filter or cen- offset by sale of end- post. Agricultural reuse by trifuge and composting. product. Distribution net - manure spreader. Landfill. DS = 30-40%. work essential. Fresh sludge and dewater- Production of very dry Very high capital costs. Agricultural reuse. Com- > 100,000 ing by belt filter or cen- product or ash. Viable if High energy consump- post. Landfill. Inciner- trifuge + drying or incin- energy source available. tion. ation. eration. DS = 80-90% (or ash).

5. Sludge treatment

Fresh sludge + thermal Minimum fuel consump- Very high capital cost. Incineration (site-specific) >300,000 conditioning + filter press tion achieved through en- High risk of odour. Spe- + incineration (ash) ergy recovery from incin- cial treatment of thermal eration. liquors. AEROBIC STABILIZED Simple Poor filterability of SLUDGE sludge. Risk of odour. Stabilized sludge and dry- Lowest capital cost. High space requirement. Agricultural reuse of liq- >10,000 ing beds. DS = 30% Uncomplicated. Limited reduction of or- uid or solid phase. Land- ganics. High labour flu. requirement for removal of dried sludge. Subject to weather conditions. Liquid stabilized sludge. Low capital cost. Uncom- H i g h s t o r ag e s p a c e Agricultural reuse of liq- < 5,000 DS = 2-5% plicated. requirement. High cost of uid product. land disposal. Stabilized sludge and Moderate capital cost. Reagent costs, although Agricultural reuse. Vis- < 15,000 drainage. DS = 5-10% Simple. Moderate storage these are offset by lower cous but pumpable prod- requirement. land disposal costs. uct. Stabilized sludge and Moderate capital cost. Reagent costs. Pasty prod- Agricultural reuse possible 10 to 100,000 dewatering on belt filter Compact facility. Easy utt not always accepted using fertilizer broadcast- or centrifuge after condi- monitoring. for landfill. er. Landfill difficult. tioning using polymers. DS = 15-20% Stabilized sludge and fil- High cake dryness. High reagent costs. Mon- Agricultural reuse possible 30 to 100,000 ter press after condition- itoring critical. Relatively using manure spreader. ing using inorganic re- high capital cost. Landfill possible agents. DS = 30-35%

Chap. 24: Municipal wastewater treatment

Table 88. Main sludge treatment processes (cont.).

Sludge type, treatment Range of use in process and dry solids Advantages Disadvantages Possible destination Europe (population)

content Stabilized sludge + dewa- Production of reusable Product stabilized twice. Commercial-grade com- > 100,000 tering by belt filter or cen- compost. Outlay difficult to recoup post. Agricultural reuse trifuge + composting. DS through product sales. possible with manure = 30-40%. Distribution network spreader. Landfill. vital. SLUDGE FROM True sludge stabilization High capital cost to com- ANAEROBIC and reduction of sludge pare with savings on dry DIGESTION weight. Safe storage. solids treatment and lower energy requirement. Digested sludge + drying Moderate capital and op- Extensive 1 a n d use . Agricultural reuse of liq- beds. DS = 30%. erating costs. Labour for removal of uid or peaty product. dried sludge. Subject to Landfill. weather conditions. Liquid digested sludge. Moderate capital and op- High storage requirement. Agricultural reuse of liq- DS = 2-3%. erating costs. High land disposal costs. uid product. Digested sludge + drain- Moderate capital cost. Low land disposal costs Agricultural reuse of vis- age. DS = 5-10%. that offset reagent costs. cous but pumpable prod- uct.

5. Sludge treatment

Digested sludge and Moderate capital cost. Reagent costs. Pasty prod- Agricultural reuse possible 40,000 to 100,000 dewatering, after polymer uct not always accepted with fertilizer broadcaster. conditioning, by belt filter for agricultural use. or centrifuge. DS = 20-25% Digested sludge + filter Good cake dryness. Little High capital cost. Signif- Agricultural reuse of 50,000 to 300,000 press and conditioning reagent use required. icant reagent costs. Mon- peary product. Landfill. using inorganic reagents. itoring critical. DS = 35-40% Digested sludge + Cher- Excellent cake dryness. Very high capital cost. Agricultural reuse of >300,000 mal conditioning + filter Little or no use of con- Risk of odour. Treatment peary product. Landfill. press. DS = 50% sumables. of thermal liquors. Strict Incineration (after break- maintenance. ing up of dumps). Digested sludge + dewa- Varying dryness of final High capital costs. Re- Agricultural reuse of pasty > 100,000 tering with belt filter of product through mixing agent costs. Fuel costs if and peaty products. centrifuge + partial dry- of dried and dewatered all sludge is dried. Humus. Landfill. Inciner- ing. DS = 20-25% and sludge. Thermal auton- ation. 80-90%. omy for dry solids of 30-40%. Digested sludge + dewa- Varying dryness of final Very high capital costs. Agricultural reuse of tering on filter press + product through mixing High reagent costs. peaty product. Humus. partial drying and re-mix- of dried and dewatered Landfill. Incineration. ing. DS = 50-60%. sludge. Thermal auton- omy.

Chap. 24: Municipal wastewater treatment

. Pumps Pump selection is also a critical

consideration. For diluted sludge, open-impeller centrifugal vortex pumps are appropriate, while positive displacement pumps (eccentric rotor or plunger type) are required for primary and thickened sludge and for sludge from flotation (see page 676). . Sludge handling

The quality of sludge dewatering facilities is often contingent upon reliable, clean and functional sludge handling procedures, appropriate to the type of sludge involved. Although conveyor belts are commonly used, viscous sludge can be pumped through pipes over short distances, using eccentric rotor positive displacement pumps, possibly equipped with charging lines. Over longer distances or when peaty sludge is involved, the matter can be conveyed in a completely dosed system known as a concrete pump, a type of plunger pump.

Trough type conveyors with endless chains are frequently used to transport sludge cakes from filter presses. Redler conveyors are

suitable for the transport of broken dumps of sludge, granules and ash. . Sludge storage

If dewatered sludge is to be stored, silos designed with steep asymmetrically sloped or countersloped sides are essential. The silos must be equipped with powerful extraction systems (scrapers and/or screw conveyors).

Depending on its composition, its degree of "freshness" and its conditioning, sludge is often a source of corrosion. It is therefore important to choose the right type of protection; stainless steel fittings are preferable.

Ambient humidity contributes to corrosion. Therefore, evaporation losses must be minimized (by handling in an enclosed area, providing hoods) and the premises must be carefully ventilated.

Foul odours can be limited at the source by minimizing the areas in which sludge is exposed to air. If odour becomes an environmental problem, then installation of an odour control system must be considered.