Sewage Sludge - The Problems Caused by Humanure
2019/4/4 12:16:40 view：
Conventional sewage treatment plants produce sludge in the primary settlement stage of treatment and a secondary, aerobic sludge in final settlement tank after the biozone.
The characteristics of the aerobic sludge vary with the type of sewage treatment process and it is often transferred back to the primary tank to assist with denitrification of the sewage as a whole, but it still has to be removed along with the anerobic sludge. The treatment and disposal of sludge amounts to a large part of the costs of operating sewage treatment plants. Land application of raw or treated sewage sludge can, in theory, significantly reduce the cost of sludge disposal as well as contributing to the nitrogen and phosphorus requirements of many crops and grasslands. Great in theory, but there are many problems.
Sewage sludge also contains pathogenic bacteria, viruses and protozoa along with other parasitic worms which can be dangerous to the health of humans, animals and plants. A World Health Organisation (1981) Report identified Salmonellae and Taenia as being of greatest concern. It is possible to reduce the numbers of pathogenic and parasitic organisms in sludge before application to the land by treating the sludge prior to spreading and the potential health risk is further reduced by the effects of climate, soil-microorganisms and time after the sludge is applied to the soil. But certain crops impose time limitations on planting, grazing and harvesting, making the time from spreading to harvesting very short.
Sewage sludge contains useful amounts of nitrogen, phosphorus and organic material. The organic matter in sludge can improve the water retaining properties and structure of some soils, making it useful for sandy land.
The characteristics of the aerobic sludge vary with the type of sewage treatment process and it is often transferred back to the primary tank to assist with denitrification of the sewage as a whole, but it still has to be removed along with the anerobic sludge. The treatment and disposal of sludge amounts to a large part of the costs of operating sewage treatment plants. Land application of raw or treated sewage sludge can, in theory, significantly reduce the cost of sludge disposal as well as contributing to the nitrogen and phosphorus requirements of many crops and grasslands. Great in theory, but there are many problems.
Sewage sludge also contains pathogenic bacteria, viruses and protozoa along with other parasitic worms which can be dangerous to the health of humans, animals and plants. A World Health Organisation (1981) Report identified Salmonellae and Taenia as being of greatest concern. It is possible to reduce the numbers of pathogenic and parasitic organisms in sludge before application to the land by treating the sludge prior to spreading and the potential health risk is further reduced by the effects of climate, soil-microorganisms and time after the sludge is applied to the soil. But certain crops impose time limitations on planting, grazing and harvesting, making the time from spreading to harvesting very short.
Sewage sludge contains useful amounts of nitrogen, phosphorus and organic material. The organic matter in sludge can improve the water retaining properties and structure of some soils, making it useful for sandy land.
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