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Sludge

Sludge

[sluhj]

Sludge is the residual semi-solid material left from industrial, water treatment, or wastewater treatment processes.

When fresh sewage or wastewater is added to a settling tank, approximately 50% of the suspended solid matter will settle out in about an hour and a half. This collection of solids is known as raw sludge or primary solids and is said to be "fresh" before anaerobic processes become active. Once anaerobic bacteria take over, the sludge will become putrescent in a short time and must be removed from the sedimentation tank before this happens.

This is commonly accomplished two ways. In an Imhoff tank, fresh sludge is passed through a slot to the lower story or digestion chamber where it is decomposed by anaerobic bacteria, resulting in liquefaction and reduced volume of the sludge. After digesting for an extended period, the result is called "digested" sludge and may be disposed of by drying and then landfilling.

Alternately, the fresh sludge may be continuously extracted from the tank mechanically and passed to separate sludge digestion tanks that operate at higher temperatures than the lower story of the Imhoff tank and, as a result, digest much more rapidly and efficiently.

Excess solids from biological processes such as activated sludge can be referred to as sludge, although more often called "biosolids," a public relations term that is increasingly used by water professionals in the United States. Industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Surface water plants also generate sludge made up of solids removed from the raw water.

Background

Biosolids, the treated form of sewage sludge, have been in use in UK and European agriculture for more than 80 years, though there is increasing pressure to stop the practice of land application. In the 1990s there was pressure in some European countries to ban the use of sewage sludge as a fertilizer. Switzerland, Sweden, Austria, and others introduced a ban. Since the 1960s there has been cooperative activity with industry to reduce the inputs of persistent substances from factories. This has been very successful and, for example, the content of cadmium in sewage sludge in major European cities is now only 1% of what it was in 1970.

European legislation on dangerous substances has eliminated the production and marketing of some substances that have been of historic concern such as persistent organic micropollutants. The European Commission has said repeatedly that the "Directive on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture" (86/278/EEC) has been very successful in that there have been no cases of adverse effect where it has been applied. The EC encourages the use of sewage sludge in agriculture because it conserves organic matter and completes nutrient cycles. Recycling of phosphate is regarded as especially important because the phosphate industry predicts that at the current rate of extraction the economic reserves will be exhausted in 100 or at most 250 years.

Treatment process

Sewage sludge is produced from the treatment of wastewater and consists of two basic forms — raw primary sludge (basically faecal material) and secondary sludge (a living ‘culture’ of organisms that help remove contaminants from wastewater before it is returned to rivers or the sea). The sludge is transformed into biosolids using a number of complex treatments such as digestion, thickening, dewatering, drying, and lime stabilisation. The more treated the wastewater the more toxic the sludge.

Benefits of treatment

The treatment process reduces the water content of the sludge. The basic principal is the cleaner the water is after the sludge is removed, the more toxic the sludge is going to be. This has been proven when random samplings of treated sludge are found to be filled with heavy metals, as well as chemical residues that are not removed by the treatment process. The treatment process does not remove 100% of the pathogens, and in many cases pathogen regrowth after spreading is significant.

Final product

Treated biosolids can be produced in cake, granular, pellet or liquid form and are spread over land before being incorporated into the soil or injected directly into the soil by specialist contractors.

Digested sewage sludge can be used as a soil conditioner, but may contain toxic materials. To be USDA-certified organic, sludge (biosolids) cannot be used. After the 1991 Congressional ban on ocean dumping, the U.S. Environmental Protection Agency (EPA) instituted a policy of digested sludge reuse on agricultural land. The EPA promoted this policy by presenting it as recycling. This practice has been highly controversial. EPA has no system to track and respond to health complaints related to exposure to sewage sludge, over three hundred and fifty people have reported sludge-based health incidents to the Cornell Waste Management Institute alone. Symptoms reported have included: asthma, weight loss, fatigue, eye irritations, flu-like symptoms, gastrointestinal complications, headaches, immunodeficiency problems, lesions, nausea, nosebleeds, rashes, respiratory complications, abscesses, reproductive complications, cysts, and tumors.

Incineration

Incineration greatly reduces the volume of the sludge and eliminates biohazard concerns. Such systems requires multi-step cleaning of the exhaust gas. However, the ash is difficult to use due to its high heavy metal content.

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