Topic4.4 hw

1. Topic4.4 hw Collected by jhp

2. Topic no4.4hw • 4.4 treatment and disposal of Sludge solids:- Aerobic , Anaerobic digestion, vacuum filtration, drying beds, drying and incineration, sanitary land fills;-

4. Thickening • Digestion with or without heat • Drying on sand bed -- open or covered • Conditioning with chemicals • Elutriation • Vacuum filtration • Heat drying • Incineration • Wet oxidation • Centrifuging

5. Sludge Processing AlternativesThickening Stabilization Conditioning Dewatering VolumeReduction Disposal 1. Gravity 1. Anaerobic 1. Chemical 1. Vacuum 1. Incineration 1. Landfill 2. Flotation 2. Aerobic 2. Thermal 2. Pressure 2. Wet Oxidation 2. Water 3. Centrifug 3. Thermal 3. Blending 3. Centrifug 3. Soil 4. Chemical 4. Drying Beds 4.Composting

6. Stabilization 1. Aerobic DigestionAerobic digestion is an extension of the activated sludge aeration process whereby waste primary and secondary sludges are continually aerated for long periods of time.  In aerobic digestion the microorganisms extend into the endogenous respiration phase, which is a phase where materials previously stored by the cell are oxidized, with a reduction in the biologically degradable organic matter.  This organic matter, from the sludge cells is oxidized to carbon dioxide, water and ammonia.  The ammonia is further converted to nitrates as the digestion process proceeds. 

7. Parameters:  Some parameters affecting the aerobic digestion process are:  (1)  rate of sludge oxidation, (2)  sludge temperature, (3)  system oxygen requirements, (4)  sludge loading rate, (5)  sludge age, and (6) sludge solids characteristics. 

8. Design:  Aerobic digestion has been applied mostly to various forms of activated sludge treatment, usually "total oxidation" or contact stabilization plants.  However, aerobic digestion is suitable for many types of municipal and industrial wastewater sludges, including trickling filter humus as well as waste activated sludges.  Any design for an aerobic digestion system should include:  an estimate of the quantity of sludge to be produced, the oxygen requirements, the unit detention time, the efficiency desired, and the solids loading rate. 

9. Detention Time:  • Waste activated sludge only, after sludge thickening. 10 - 15 days volumetric displacement time. If sludge temperatures are much less than 60°F, more capacity should be provided.  • Primary sludge mixed with waste activated or trickling filter humus. 20 days displacement time in moderate climates. 

10. The advantages most often claimed for aerobic digestion are: 1 A humus-like, biologically stable end product is produced.  • The stable end product has no odors, therefore, simple land disposal, such as lagoons, is feasible.  • Capital costs for an aerobic system are low, when compared with anaerobic digestion and other schemes.  • Aerobically digested sludge usually has good dewatering characteristics.  When applied to sand drying beds, it drains well and redries quickly if rained upon.  • The volatile solids reduction can be equal to those achieved by anaerobic digestion.  • Supernatant liquors from aerobic digestion have a lower BOD than those from anaerobic digestion.  Most tests indicated that BOD would be less than 100 ppm.  This advantage is important because the efficiency of many treatment plants is reduced as a result of recycling high BOD supernatant liquors.  • There are fewer operational problems with aerobic digestion than with the more complex anaerobic form because the system is more stable.  As a result, less skillful labor can be used to operate the facility.  • In comparison with anaerobic digestion, more of the sludge basic fertilizer values are recovered. 

11. Co-composting - solid sludge • Mixing of hazardous wastes with biodegradable solids to act as: • bulking agent - to create void spaces for passage of air • thermal source - by biological decomposition • Usually need two different materials as: • good bulking agent eg wood chips are poor thermal source • good thermal source eg dry molasses are poor bulking agent • Process takes place: • in windrows, turned to ensure adequate aeration • in static piles where air is forced or sucked through material • in vessel which offers greater process control/ VOC containment • Used for: • soils contaminated with coal tar • for TNT-contaminated sediments and soils

12. The major disadvantage associated with aerobic digestion is high power costs.  This factor is responsible for the high operating costs in comparison with anaerobic digestion.  At small waste treatment plants, the power costs may not be significant but they certainly would be at large plants.  Some investigators have observed that aerobically digested sludge does not always settle well in subsequent thickening processes.  This situation leads to a thickening tank decant having a high solids concentration. 

13. 2. Anaerobic DigestionThe purpose of digestion is to attain both of the objectives of sludge treatment -- a reduction in volume and the decomposition of highly putrescible organic matter to relatively stable or inert organic and inorganic compounds.  Additionally, anaerobic sludge digestion produces a valuable by-product in the form of methane gas. 

14. Anaerobic digestion is a complex biochemical reaction carried out in a number of steps by several types of micro-organisms that require little or no oxygen to live. During the process, a gas principally composed of methane (CH4) and carbon dioxide (CO2),....otherwise known as biogas, is produced. The amount of gas produced varies with the amount of organic waste fed to the digester and temperature influences the rate of decomposition (and gas production).

15. General Description.   Sludge digestion is carried out in the absence of free oxygen by anaerobic organisms.  It is, therefore, anaerobic decomposition.  The solid matter in raw sludge is about 70 percent organic and 30 percent inorganic or mineral.  Much of the water in wastewater sludge is "bound" water which will not separate from the sludge solids.  The facultative and anaerobic organisms break down the complex molecular structure of these solids setting free the "bound" water and obtaining oxygen and food for their growth.  Anaerobic digestion involves many complex biochemical reactions and is dependent on many interrelated physical and chemical factors.  For purposes of simplification, the anaerobic degradation of domestic sludges occurs in two steps. 

18. Anaerobic digestion occurs in four distinct steps, illustrated in Previous Figure . Hydrolysis: complex organic matter is decomposed into simple soluble organic molecules using water to split the chemical bonds between the substances.Fermentation or acidogenesis: the chemical decomposition of carbohydrates by enzymes, bacteria, yeasts, or molds in the absence of oxygen.Acetogenesis: the fermentation products are converted into acetate, hydrogen and carbon dioxide by so-called acetogenic bacteria.Methanogenesis: methane (CH4) is formed from acetate and hydrogen/carbon dioxide by methanogenic bacteria. .The acetogenic bacteria grow in close association with the methanogenic bacteria (Figure II-2) during the fourth stage of the process. The reason for this is that the conversion of the fermentation products by the acetogens is thermodynamically only possible if the hydrogen concentration is kept sufficiently low. This requires a close symbiotic relationship between both classes of bacteria.

24. Anaerobic granular sludge was used to treat hazardous chlorinated hydrocarbons,such as pentachlorophenol (PCP), tetrachloroethylene (PCE) and trichloroethylene (TCE). Granular sludge capable of reductive dechlorination of the above chltorinaed compounds was developed in laboratory-scale anaerobic reactors.using microbial cultures enriched with these chlorinated compounds as inocula. The PCP-degrading granular sludegdechlorinated and then degraded PCP into methane and CO2. The PCE/TCE-dechlorinating granular sludge dechlorinated PCE.TCE and other chlorinated ethylenes into ethylene. These dechlorinating granular sludge could be applied in the treatment of wastewater and groundwater contaminated with chlorophenols and chlorinated ethylene. Anaerobic granular sludge was used to treat hazardous chlorinated hydrocarbons,such as pentachlorophenol (PCP), tetrachloroethylene (PCE) and trichloroethylene (TCE). Granular sludge capable of reductive dechlorination of the above chltorinaed compounds was developed in laboratory-scale anaerobic reactors.using microbial cultures enriched with these chlorinated compounds as inocula. The PCP-degrading granular sludegdechlorinated and then degraded PCP into methane and CO2. The PCE/TCE-dechlorinating granular sludge dechlorinated PCE.TCE and other chlorinated ethylenes into ethylene. These dechlorinating granular sludge could be applied in the treatment of wastewater and groundwater contaminated with chlorophenols and chlorinated ethylene.

29. Thickening 1. Gravity Thickening This process involves the concentration of thin sludges to more dense sludge in special circular tanks designed for this purpose.  Its use is largely restricted to the watery excess sludge from the activated sludge process, and in large plants of this type where the sludge is sent direct to digesters instead of to the primary tanks.  It may also be used to concentrate sludge to primary tanks or a mixture of primary and excess activated sludge prior to high rate digestion. The thickening tank is equipped with slowly moving vertical paddles built like a picket fence.  Sludge is usually pumped continuously from the settling tank to the thickener which has a low overflow rate so that the excess water overflows and the sludge solids concentrate in the bottom.  A blanket of sludge is maintained by controlled removal which may be continuous at a low rate.  A sludge with a solids content of ten percent or more can be produced by this method.  This means that with an original sludge of two percent, about four-fifths of the water has been removed, and one of the objectives in sludge treatment has been attained. 

30. 2. Flotation Thickening General -- Flotation thickening units are becoming increasingly popular at sewage treatment plants, especially for handling waste activated sludges.  With activated sludge they have the advantage over gravity thickening tanks of offering higher solids concentrations and lower initial cost for the equipment.  Dissolved Air-Pressure Flotation Theory:  The objective of flotation-thickening is to attach a minute air bubble to suspended solids and cause the solids to separate from the water in an upward direction.  This is due to the fact that the solid particles have a specific gravity lower than water when the bubble is attached. 

31. Dissolved air flotation depends on the formation of small diameter bubbles resulting from air released from solution after being pressurized to 40 to 60 psi.  Since the solubility of air increases with pressure, substantial quantities of air can be dissolved.  In current flotation practice, two general approaches to pressurization are used:  (1)  air charging and pressurization of recycled clarified effluent or some other flow used for dilution, with subsequent addition to the feed sludge; and (2)  air charging and pressurization of the combined dilution liquid and feed sludge.  Air in excess of the decreased solubility, resulting from the release of the pressurized flow into a chamber at near atmospheric pressures, comes out of of solution to form the minute air bubbles.  Sludge solids are floated by the air bubbles that attach themselves to and are enmeshed in the floc particles.  The degree of adhesion depends on surface properties of the solids.  When released into the separation area of the thickening tank, the buoyed solids rise under hindered conditions analogous to those in gravity settling and can be called hindered separation or flotation.  The upward moving particles form a sludge blanket on the surface of the flotation thickener. 

32. Parameters:  The primary variables for flotation thickening are:  (1)  pressure, (2)  recycle ratio, (3)  feed solids concentration, (4)  detention period, (5)  air-to-solids ratio, (6)  type and quality of sludge, (7)  solids and hydraulic loading rates, and (8)  use of chemical aids. 

33. Similar to gravity sedimentation, the type and quality of sludge to be floated affects the unit performance.  Flotation thickening is most applicable to activated sludges but higher float concentrations can be achieved by combining primary with activated sludge.  Equal or greater concentrations may be achieved by combining sludges in gravity thickening units. 

34. 3.  Centrifugation Centrifugation has been demonstrated to be capable of thickening a variety of waste sludges