There are multitudes of aerobic biological treatment processes and technologies in literature and practice. Biological treatment using aerobic activated sludge process has been in practice for long.
Wastewater treatment has become essential these days, because it is so interconnected with the other uses of water. This wastewater is treated and then converted to effluent to discharge it to the natural water streams. If un-treated water discharged in river streams, leave its effects on the wildlife habitats thriving in oceans, rivers and marshes, migratory birds using these areas for breeding, resting, and nesting, fisheries which have direct impact upon human consumption. Thus this effluent must be treated before it goes back to the environment to have minimum impact on the environment, or can be directly reused. This reclaimed water can be used for purposes other than consumption. As disposal or reuse is the ultimate aim of the treatment of wastewater, treatment is decided accordingly to create minimum impact to the river streams and low sludge for landfills.
Wastewater is treated in three stages primary, secondary and tertiary. Primary treatment includes filtering the insoluble solids, grit, suspended matter etc. from water, the sand filter is the most commonly used filter. The oil and grease found on the surface of some wastewater can also be removed easily through this method. Next is secondary treatment, the collected filtered water is then treated to reduce its toxicity, decrease oxygen demand and disinfect. This treatment includes oxidation and chemical treatment. Oxidation can be aerobic or anaerobic involved in treatment and the tertiary treatment involves further disinfection like absorption, advanced oxidation, disinfection depending upon quality of wastewater.
The oxidation is the most important and integral part of the wastewater treatment processes, it defines the efficacy of the treatment plant. Thus, secondary treatment should be efficient as well as it should be economical. Oxidation means decomposition of organic or inorganic matters present in effluent. The secondary treatment involves biological oxidation and chemical oxidation. Biological oxidation processes can be aerobic and anaerobic. Rely on microbial decomposition to treat wastewater, the key difference between anaerobic and aerobic treatment is that aerobic systems require oxygen, while anaerobic systems do not. Most people consider bacteria and other microorganisms to be undesirable components of wastewater. In fact, only a small fraction of the microbes found in wastewater are truly pathogenic. Aerobic wastewater treatment encourages the growth of naturally-occurring aerobic microorganisms as a means of renovating wastewater. Such microbes are the engines of wastewater treatment plants. Organic compounds are high-energy forms of carbon. The oxidation of organic compounds to the low-energy form (carbon dioxide) is the fuel that powers these engines.
The biological aerobic treatment uses bacteria or microbes to clean water systems that degrade the organic matter in presence of oxygen. The oxygen required for the decomposition of organic matter by these microbes is often measured in biological oxygen demand or BOD, which refers to the amount of dissolved oxygen needed by aerobic organisms to break down organic matter into smaller molecules. Provided that oxygen and food, in the form of settled wastewater are supplied to the microorganisms, the biological oxidation process of dissolved organic matter will be maintained. The bioconversion of dissolved organic matter into thick bacterial biomass can fundamentally purify the wastewater. Subsequently, it is crucial to separate the microbial biomass from the treated wastewater through sedimentation. This secondary sedimentation is basically similar to primary sedimentation except that the sludge contains bacterial cells rather than fecal solids. The biological removal of organic matter from settled wastewater is conducted by microorganisms, mainly heterotrophic bacteria but also occasionally fungi. The microorganisms are able to decompose the organic matter through two different biological processes: biological oxidation and biosynthesis. The biological oxidation forms some end-products, such as minerals that remain in the solution and are discharged with the effluent. The biosynthesis transforms the colloidal and dissolved organic matter into new cells that form in turn the dense biomass that can be then removed by sedimentation.
COHNS + O2 + Bacteria → CO2 + NH3 + Other end products
(Organic matter) +Energy
COHNS + O2 + Bacteria → C5H7NO2
(Organic matter) (New cells)