SLUDGE TREATMENT AND DISPOSAL

Transcription

1 SLUDGE TREATMENT AND DISPOSAL General There are two end products of the waste water treatment. 1. The treatment plant effluent - Directly discharged in the natural water sources or into receiving soils. 2. The sludge - Contains highly putrescible substances and pathogenic organisms. - Treated further for safe disposal Sludge and Its Moisture Content Raw Sludge - Deposited in a primary sedimentation tank. - Odorous and contains highly putrescible matter very objectionable. - Moisture content 95%. Secondary Sludge - Deposited in a secondary clarifier. - Contains putrescible matter, but less objectionable compared to raw sludge. - Moisture content 96% - 98% (trickling filter) and 98% - 99% (activated sludge plant) Problem: A sedimentation tank is treating 4.5 million litres of sewage per day containing 275 ppm of suspended solids. The tank removes 50% of suspended solids. Calculated the quantity of sludge produced per day in bulk and weight, if (a) moisture content of sludge is 98% ; (b) moisture content of sludge is 96%. Solution: Given: volume of sewage treated = 4.5 x 10 6 L/day Suspended solids present = 275 ppm = 275mg/L = 275 x 10-6 kg/l ie., In 1 L of sewage 275 x 10-6 kg of suspended solids is present. 4.5 x 10 6 L the mass of suspended solids present = 275 x 10-6 (kg/l) x 4.5 x 10 6 (L/day) = kg/day Also given, % of solids removed in sedimentation tank = 50 % Thus, the mass of solids removed in sedimentation tank = 50% of kg/day = kg/day 5 Case (a) : moisture content of sludge is 98% ie., 2kg of solids or dry sludge makes 100 kg of wet sludge. Thus, mass of wet sludge made by kg of solids = kg ~ kg Assume, specific gravity of wet sludge = 1.02 Unit weight of water = 1000kg/m3 So, unit weight of wet sludge 1

2 = 1.02 x 1000 kg/m3 = 1020kg/m3 6 Case (b) : moisture content of sludge is 96% Weight of sludge = kg of wet sludge Volume of sludge= cu.m Sludge Digestion Process The process of stabilization of sludge withdrawn from the sedimentation basin. - By decomposing the organic matter under controlled anaerobic conditions. - 40% - 60% of organic solids converted into CO 2 and CH 4 gas. - Remaining organic matter will be chemically stable and odourless with 90% - 95% of moisture content. This process reduces the sludge into three forms 1. Digested sludge 2. Supernatant liquor 3. Gases of decomposition 1. Digested sludge - Stable - Humus like solid mater - Tarry black in colour - Musty earthy odour - With reduced moisture content - Volume= one third times the undigested sludge - Free from pathogenic bacteria - But may contain cysts and eggs of bacteria, worms and protozoa - Dewatered, dried up and used as fertilizer 2. Supernatant liquor - Liquified finely divided solid matter - BOD about 3000ppm - Contains 1500ppm to3000ppm of suspended solids, so re-treated with the raw sludge. 3. Gases of decomposition % CH 4-30% CO 2 - Traces of inert gases like N, H 2 S - Collected and used as fuel Stages in Sludge Digestion Process 1. Acid fermentation/acid production 2

3 2. Acid regression 3. Alkaline fermentation 1. Acid fermentation/acid production -Anaerobic and facultative bacteria (acid formers) acts. -Acid formers stabilize the organic solids through hydrolysis. -Soluble products fermented to volatile acids and organic alcohols of low molecular weight. -Evolution of CH 4, CO 2 and H 2 S gases. -Lowers ph value to less than 6 highly acidic in nature. -Evolution of highly putrescible odours. -Continues for 15 days. 2. Acid regression -Volatile organic acids and nitrogenous compounds of the first stage acted upon by bacteria. -Forms acid carbonates and ammonia compounds. -Evolution of H 2 S and CO 2 gases in small amount. -Offensive odour -ph value rises to 6.8 -Entraps of gases of decomposition and forms formy scum layer. -Continues for 3 months. -BOD remains high. 3. Alkaline fermentation -Proteins and organic acids attacked and broken up by anaerobic bacteria (methane formers). -Forms ammonia, organic acids and gases. -Liquid separates out from solids and the digestive sludge (or ripened sludge) is formed. -Digestive sludge collected at the bottom of digestion tank. -Alkaline in nature ph value rises little above 7. -Large amount of CH4 and small amount of CO2 and N gases are evolved. -Continues for 1 month. -BOD rapidly falls down. Factors Affecting Sludge Digestion Process 1. Temperature 2. ph value 3. Seeding with digested sludge 4. Mixing and stirring of raw sludge with digested sludge1 3

4 Steps of Sludge Treatment a) Thickening b) Conditioning c) Dewatering d) Drying e) Incineration f) Disposal a) Thickening - Reduces moisture content of sludge, thereby reducing volume of sludge. - Three types of thickening 1. Gravity thickening 2. Air floatation 3. Centrifugation b) Conditioning - Conditioning improves the drainability of digested sludge. - Methods adopted 1. Elutriation (involves passage of air to separate particles) 2. Chemical conditioning (addition of coagulants) 3. Freezing etc. 4

5 c) Dewatering The digested sludge from the digestor contains lots of water which should be removed by dewatering and then disposed off. i. Sludge drying beds ii. Mechanical methods i. Dewatering, drying and disposal of sludge by sludge drying beds. - Drying of the digested sludge on open beds of land sludge drying beds. - Suitable for hot countries India. ii. Mechanical methods of dewatering sludge - By vacuum filtration or by high speed centrifuges. - 50% moisture removed - High speed centrifuges requires only a small area. - Before vacuum filtration the sludge is washed by the process of elutriation to remove the organic and fatty acids. d) Drying Reduces water content 350 o C Process carried out in kiln. Dried sludge used as soil conditioner 5

6 e) Incineration burning sludge at high temperatures in furnace volume is reduced ash is stabilized requires air permit & pollutant removal high operating costs complex 650 C to 750 C 6

7 f) Sludge Reuse or Disposal Options Landfill disposal of sludge in a lined landfill leachate collection required odor potential local regulatory requirements inexpensive Trucking (Transportation) costs Surface disposal trench, waste pile, lagoon biosolids remain longer than 2 years surface impoundment dedicated disposal sites Land Application application of sludge to condition the soil or fertilize crops / other vegetation sprayed or spread on soil surface tilled into soil or injected directly below the surface must be applied at agronomic rates, meet pollutant limits, and pathogen & vector attraction reduction requirements requires 30 days or more of storage Disposal by dumping into sea 7