Composting Process

Composting of wastewater sludge is an aerobic biothermal process that decomposes the organic constituents. It can be described by the formula C6H12O6 + 6O2 6CO2 + 6H2O + 674 kcal It can also be performed as an anaerobic process described by the formula The aerobic process provides a higher caloric content. It goes much faster than the anaerobic process, and decomposition of the organic constituents produces a stable humuslike material. Microbiology Composting represents the combined activity of...

Dry Lime Mixing

Mixing of dewatered sludge cake and lime (or other alkaline materials) is the most critical component of dry lime stabilization. The goal is to provide intimate contact between the sludge cake and lime and to produce a homogenized product. The mixing step is typically accomplished using a mechanical mixer such as a pug mill, plow blender, paddle mixer, or screw conveyor. Dewatered sludge and lime are introduced at the head end of the mixer. Sludge cake with a low moisture content and in large...

Operational Considerations

Compared to anaerobic digestion systems, an aerobic digestion process is relatively simple to operate. Similar to an activated sludge system, the aerobic digestion process is essentially self-sustaining. Operational considerations include pH reduction, foaming problems, supernatant quality, and dewater-ability of digested sludge. pH Reduction Decreases in pH and alkalinity have been observed in aerobic digesters at increasing detention times. The drop in pH is caused by acid formation that...

Pressure Filter Press

Pressure filter press dewatering is a batch process in which dewatering is achieved by forcing the water from the sludge under high pressure. It produces a cake that is drier than that produced by any other dewatering alternative. Another advantage is that the high solids capture results in good filtrate quality. Disadvantages include high capital cost, relatively high operation and maintenance costs, high chemical costs, and a large area requirement for the equipment in small wastewater...

Belt Filter Press

Disk Belt Filter Press

A belt filter press (BFP) is a continuous-feed sludge dewatering machine with two porous moving belts that has a gravity drainage zone and mechanically applied pressure zones. Belt filter presses have been used in Europe first for dewatering paper pulp and later modified to dewater wastewater sludge. It was introduced in North America in the mid-1970s, mainly because of its ability to dewater secondary sludge economically, and its reduced energy requirements compared to centrifuges and vacuum...

System Design Considerations

Factors that must be considered in designing conventional aerobic digesters include method of operation, tank volume and detention time, tank design, and aeration and mixing equipment. Typical design criteria for aerobic digestion are presented in Table 4.1. Method of Operation The two primary operational modes for conventional aerobic digesters are batch or continuous operation, referring to the manner in which supernatant is withdrawn from the process. Figure 4.3 shows cross sections of...

Composting Mixture

The new technology requires quicklime to be mixed with the dewatered sludge just prior to adding a bulking agent (sawdust, wood chips, bark, etc.) and recycled compost. Refer to Figure 7.13 for the schematic of the proposed 1.Hopper with conveyor for dewatered sludge 2.Silos for quicklime with processing unit and pneumatic pumping of lime 3.Hopper for bulking agents and recycled compost 6.Discharge conveyor for mixture to be composted 11. Cover over composting piles 1.Hopper with conveyor for...

Examples Of Composting In Europe

Several examples of different types of composting systems that have been in use in Europe are described below. All these systems are open-air processes, mostly on asphalt pavements. In Finland, two wastewater treatment plants (city of Lappeenranta and city of Loensiy) compost sludge mixed with ground bark as the bulking agent at a ratio of 1 1 by volume in 3-m-high windrow piles. Every three weeks, piles are shoveled over by scoop loader. After two weeks of processing the temperature rises to...

Composting Process Control

Experiments show that the type and population of microorganisms varies during the composting process. It is therefore critical to control the composting environment so that the microorganisms can flourish. The composting environment parameters include the compost pile temperature, moisture content of the compost, oxygen and carbon dioxide levels in the compost pile, and the availability of nutrients, including carbon, nitrogen, phosphorus, and potassium for the microorganisms. These parameters...

References

Barrett, R., and Herndon, J. (2005), Safety First Key Safety Considerations When Drying Biosolids to Class A Specifications, Water Environment and Technology, Vol. 17, No. 4, p. 35. Craven, A., et al. (2004), Largo's Beneficial Use of Biosolids, Florida Water Resources Journal, April, pp. 42-44, 46, 48. Currents, a U.S. Filter newsletter (2002), Vol. 1, No. 1, September. Karmazinov, F. V. (Ed.) (2002), Wastewater Treatment in Saint Petersburg, New Magazine, Saint Petersburg, Russia. Kowalski,...

Examples Of Composting In The United States

In the United States, studies of windrow and aerated static pile composting were conducted in the mid-1970s in Beltsville, Maryland, and in Carson, California. As of 2003, there were more than 200 wastewater sludge composting facilities in the United States. Most of the composting facilities are aerated static pile systems, and some are in-vessel systems. Windrow composting is rarely used because of the potential odor generation and the large area requirement. The city of Eustis, Florida, uses...

Drying Beds

Sand Drying Bed

Drying beds are the most widely used method of municipal wastewater sludge dewatering in the United States. They have been used for more than 100 years. Although the use of drying beds might be expected in small plants and in warmer, sunny regions, they are also used in several large facilities and in northern climates. In the United States, a majority of wastewater treatment plants with less than 5-mgd capacity use drying beds for biosolids dewatering. In Russia and other Eastern European...

Process Description 721 Factors Influencing Composting

Composting represents the combined activity of a succession of mixed populations of bacteria, actinomycetes, and fungi at different stages of the process. The principal factors that affect the biology of composting are moisture, temperature, pH, nutrient concentration, and oxygen supply. Moisture Decomposition of organic matter depends on moisture. Less than 40 moisture may limit the rate of decomposition. The optimum moisture content is 50 to 60 . Moisture content is also important for the...

Theoretical Aspects Of Composting

The Composting Process

For the best results in a composting process, it is important to have appropriate mixing of sludge cake with bulking agents and recycled materials. For the process to operate in good condition, it needs to have the optimum mass balance, moisture, temperature, pH, nutrients, and air. Figure 7.11 shows a mass balance diagram that can be used for all three composting systems windrow, aerated static pile, and in-vessel. In the diagram A' weight of ready compost in one day A weight of recycled...

Advantages and Disadvantages of Composting

The main advantages of sludge composting are the following Compost has an abundance of nutrients and is suitable for a wide variety of end uses, such as landscaping, topsoil blending, and growth media. Compost has less nitrogen than biosolids from other stabilization processes, due to the loss of ammonia during composting. However, nitrogen in compost is released more slowly and is available to plants over a long period of time, which is more consistent with plant uptake needs. Well-composted...