Sewage and residues

The residue that accumulates in sewage treatment plants is termed sludge (or biosolids). Sewage sludge would be the solid, semisolid, or slurry residual material that's produced like a by-product of wastewater treatment processes. This residue is normally classified as primary and secondary sludge. Primary sludge is generated from chemical precipitation, sedimentation, along with primary processes, whereas secondary sludge would be the activated waste biomass caused by biological treatments. Some sewage plants also receive septage or septic tank solids from household on-site wastewater treatment systems. Quite often the sludges are combined together for even more treatment and disposal.

Treatment and disposal of sewage sludge are major factors inside design and operation of most wastewater treatment plants. Two basic goals of treating sludge before final disposal will be to reduce its volume as well as stabilize the organic materials. Stabilized sludge don't even have an offensive odour which enables it to be handled without causing a nuisance or health risk. Smaller sludge volume cuts down on costs of pumping and storage.
Treatment methods

Treatment of sewage sludge might include a combination of thickening, digestion, and dewatering processes.
Thickening

Thickening is generally the first step in sludge treatment as it's impractical to take care of thin sludge, a slurry of solids suspended in water. Thickening is normally accomplished in a very tank termed as a gravity thickener. A thickener is able to reduce the total number of sludge to less the original volume. An alternative to gravity thickening is dissolved-air flotation. In this method, air bubbles carry the solids for the surface, the place where a layer of thickened sludge forms.
Digestion

Sludge digestion is really a biological process through which organic solids are decomposed into stable substances. Digestion cuts down on the total mass of solids, destroys pathogens, and makes it easier to dewater or dry the sludge. Digested sludge is inoffensive, finding the appearance and characteristics of the rich potting soil.

Most large sewage treatment plants employ a two-stage digestion system where organics are metabolized by bacteria anaerobically (within the absence of oxygen). In the initial stage, the sludge, thickened into a dry solids (DS) content around 5 percent, is heated and mixed in the closed tank for several days. Acid-forming bacteria hydrolyze large molecules for instance proteins and lipids, breaking them into smaller water-soluble molecules, then ferment those smaller molecules into various fat. The sludge then flows to a second tank, the place that the dissolved matter is converted by other bacteria into biogas, an assortment of carbon dioxide and methane. Methane is combustible which is used like a fuel to heat the primary digestion tank in addition to generate electricity to the plant.

Anaerobic digestion is quite sensitive to temperature, acidity, along with other factors. It requires careful monitoring and control. In some cases, the sludge is inoculated with extra hydrolytic enzymes at the beginning of the primary digestion stage so that you can supplement the action of the bacteria. It has been discovered that this enzymatic treatment can destroy more unwanted pathogens inside sludge and as well can result inside the generation of more biogas from the second stage of digestion.