Reed Drying Beds

Reed bed drying technology is applicable to many small wastewater treatment plants. Reed beds use common reed plants such as phragmites australis or phragmites

Table 15 Conventional sludge dewatering operations

Unit operation

Equipment

Achievable solid concentration

Limitation

Centrifugation

Basket and bowl centrifuges

Waste AS: 10-20% Mixture: 15-30% Primary sludge: 30-35%

Difficult to remove fines in centrate Not cost-effective

Filtration

Vacuum filter

Waste AS: 12-18% Primary sludge: 25-30%

Poor results on waste AS

Pressure filter

Waste AS: 20-25% Primary sludge: 50%

High pressure (689-1380 kPa) May need chemical conditioning

Belt-press filter

20-30%

Membrane filter

New but less mature

Sand filter

Small systems in dry areas Not cost effective

Data from Perry RH, Green DW, and Maloney JO (eds.) (1997) Perry's Chemical Engineer's Handbook, 7th edn. New York: McGraw Hill, and Chen G, Yue PL, and Mujumdar AS (2002) Sludge dewatering and drying. Drying Technology 20: 883 916.

Data from Perry RH, Green DW, and Maloney JO (eds.) (1997) Perry's Chemical Engineer's Handbook, 7th edn. New York: McGraw Hill, and Chen G, Yue PL, and Mujumdar AS (2002) Sludge dewatering and drying. Drying Technology 20: 883 916.

Table 16 Main sludge drying processes

Process

Equipment

Performance: solid concentration (SC) or drying rate (DR) or evaporation rate (EV)

Limitation

Solar drying

Solar energy collector

SC: 35%

1. Large land

2. Odor problem

Direct drying

Rotary-drum dryer

EV: 800-50 000 kg h 1

1. Development of odor and

Cage-mill flash dryer

EV: 6000-8000 kg h 1

toxicity hazard

Belt dryer

SC: 70-80%

2. Formation of sticky plastic

phase

3. Not cost effective

4. Gases not easily separable

and condensable

Indirect drying

Horizontal agitated thin film

SC: 65%

As above except point 4

evaporator

DR: 20-160 kg m 2h 1

Rotary paddle or disc dryers

SC: >90%

Combined

Fluidized-bed dryer

SC: 95%

As above

drying systems

Efficiency improvement

Carver-

Multiple effect evaporator

SC: >97%

Complex multi-unit operations

Greenfield

Hydro-extractor

Odor and pollution free

process

Low heat energy

Requirement of solvent

Production of clean fuel oil

Drying and pellet

Vertical multistage

SC: 95%

Relatively complex operations

production

tray dryer

with low valued products

Drying and

Rotary kiln and integrated

Development of thermal energy and

Popular in Japan, but not

incineration

fluidized bed dryer/

micro-size particles

widely adopted worldwide

incinerator

Data from KasakuraT, Imoto Y, and Mori T (1993) Overview and system analysis of various sewage sludge drying processes. Drying Technology 11: 871 900; Chen G, Yue PL, and Majumdar AS (2002) Sludge dewatering and drying. Drying Technology 20: 883 916; and Trowbridge TD and Holcombe TC (1996) for waste treatment The Carver Greenfield process: Dehydration/solvent extraction technology. Environmental Progress 15: 213 220.

Data from KasakuraT, Imoto Y, and Mori T (1993) Overview and system analysis of various sewage sludge drying processes. Drying Technology 11: 871 900; Chen G, Yue PL, and Majumdar AS (2002) Sludge dewatering and drying. Drying Technology 20: 883 916; and Trowbridge TD and Holcombe TC (1996) for waste treatment The Carver Greenfield process: Dehydration/solvent extraction technology. Environmental Progress 15: 213 220.

communis to evaporate water in a confined area. The selected reeds are planted in sand beds with any conve nient shape to accommodate the existing land conditions and space. Well stabilized and volatized sludges are poured on the reed beds every few weeks during the growing season. Reeds accomplish the task of drying sludges mainly through transpiration. The advantages of the reed bed drying technology include minimal capital investment and low day to day operational cost. A major disadvantage of the technique is that, reed plants may be very invasive in many farming or natural environments. Consequently, the key research challenge of reed drying beds relate to methods to prevent escape of the reed.

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