In this article we will discuss about:- 1. Elements of Sewage (or Wastewater) Management System 2. Necessity of Sewage (or Wastewater) Treatment 3. Requirements of a Sewage (or Wastewater) Management System.
Elements of Sewage (or Wastewater) Management System:
The various elements of sewage (or wastewater) management system are as indicated below:
1. Sources of sewage (or wastewater)
2. On-site processing
3. Collection of sewage (or wastewater)
4. Transmission of sewage (or wastewater)
5. Treatment of sewage (or wastewater)
6. Disposal of treated sewage (or wastewater)
A brief description of each of these elements is given below:
1. Sources of Sewage (or Wastewater):
The various sources of sewage (or wastewater) in a community are residences, commercial establishments and industries.
2. On-Site Processing:
The on-site processing involves facilities for pretreatment or flow equalisation of sewage (or wastewater) before it is discharged to a collection system. It is primarily necessary for industrial sewage (or wastewater).
3. Collection of Sewage (or Wastewater):
It involves facilities for collection of sewage (or wastewater) from individual sources in a community.
4. Transmission of Sewage (or Wastewater):
It involves facilities to pump and transport collected sewage (or wastewater) to processing and treatment sites.
5. Treatment of Sewage (or Wastewater):
It involves facilities for treatment of sewage (or wastewater).
6. Disposal of Treated Sewage (or Wastewater):
It involves facilities for disposal of treated effluent and residual solids resulting from treatment.
Necessity of Sewage (or Wastewater) Treatment:
The necessity of the sewage (or wastewater) treatment, before its final disposal, arises because of the putrescible organic matter present in the sewage. The organic matter undergoes decomposition due to bacteria and it is this decomposition that causes nuisance and troubles. Sewage (or wastewater) is treated to eliminate or to a greater extent reduce its nuisance characteristics.
The treatment of sewage (or wastewater) is divided into five parts indicated below:
i. Preliminary treatment
ii. Primary treatment
iii. Secondary treatment or biological treatment
v. Disposal of sewage sludge
Preliminary treatment consists of removing floating material and heavy settle able inorganic solids by screening. The action in this case is purely mechanical.
Primary treatment consists of removing suspended organic solids by sedimentation. However, if preliminary treatment is not given, then primary treatment removes both inorganic as well as organic solids. The action in this case is also purely mechanical.
Secondary treatment also called biological treatment consists of removing organic matter by biological processes involving aerobic as well as anaerobic bacteria.
Sometimes sewage (or wastewater) is also given tertiary treatment (also known as advanced treatment) which consists of removing the pollutants not removed in primary and secondary treatment. These pollutants may include soluble inorganic compounds (such as phosphorous and nitrogen) remaining organic matter, pathogens, colloidal solids, soluble minerals and other toxic substances.
Disinfection consists of applying chlorine to sewage (or wastewater) in order to destroy the disease producing organisms.
The primary treatment of sewage (or wastewater) gives rise to sewage sludge which is accumulated in the sedimentation or settling tank. The sewage sludge needs to be treated before its final disposal. The treatment of sewage sludge involves its digestion or decomposition under the influence of anaerobic bacteria in the absence of oxygen.
The sludge digestion produces sludge gas and digested sludge. Sludge gas can be utilized as fuel or for generating power, etc. Digested sludge is subjected to dewatering and drying processes, either natural or chemical and mechanical before final disposal as fill or for use as a soil conditioner.
Sludge after dewatering and drying may also be burnt and ash used for fill. It may however, be stated that treatment and disposal of sewage sludge is a major economic factor since the cost of its treatment and disposal is about one-third of that involved in the entire process of treatment of sewage (or wastewater).
The various requirements of a sewage (or wastewater) management system are as indicated below:
1. The system should be such that there is no health hazard resulting from the various works of the system.
2. The method adopted for the treatment of sewage (or wastewater) should be such that a high degree of destruction of pathogens is achieved and various harmful chemical pollutants are removed or neutralized.
3. The treatment method adopted should be such that the treated effluent is safe to be reused for aquaculture, agriculture, industrial cooling and groundwater recharge. However, if the treated effluent is not reused but it is disposed of in a natural stream or river, its characteristics should be such that it would not pollute the water in the receiving stream.
4. The treatment processes of the system should be such that no objectionable odour is released and nuisance on this account is avoided.
5. The system should be such that it may be operated with minimum difficulties.
6. The system should have minimum possible capital and operational costs. To some extent the system may be rendered economical if the end products are reused. However, if the end products are not reused, their disposal should be achieved with minimum possible cost so as to economize the entire system.