Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treatment, and numerous other industrial processes.
For gasification to take place the sludge is usually transformed into biosolids using a number of complex treatments such as digestion, thickening, dewatering, drying, and lime stabilization. Sewage sludge can be composted and it can be spread on land. However, it also unfortunately is known to contain numerous organic micro pollutants, and pathogens. Apart from substantial direct health risks for humans. This sludge, if applied to land, also results in contamination of the environment due to the presence of ammonia, and BOD etc. It is in fact an amalgamation of most the wastes of our society and it is not closely controlled so it may contain highly undesirable polluting trace contaminants.
Sewage sludge is an inevitable and unavoidable by-product of sewage treatment. The amount produced is massive and is also expected to rise by rapidly in Europe in particular, mainly as a result of the higher treatment standards provided through the UWWTD.
To achieve gasification sewage sludge is placed into a dewatering or dehydration chamber. The low water content sludge cake remaining is heated to convert moisture in the sludge to steam and cause gasification. These facilities are usually large because sewage sludge is produced in huge quantities day after day, year after year. In fact the municipalities find themselves under relentless pressure to get rid of the stuff.
Sewage sludge is primarily made of water, and 90 to 95 percent of its mass can be eliminated through the thermal drying process. The cost of thermal drying is usually easily repaid as the remaining 5 to 10 percent is much less expensive to transport and burn than the original material leaving waste water treatment tanks.