Nearly half of the population in Massachusetts channel their sewage to MWRA at the Boston harbour for treatment. Waste reaches the plant through three different pipes that function mostly by force of gravity or by pumps in low altitude regions. At the sewage plant the waste undergoes two treatment processes categorized as primary and secondary (Deer Island). However, before treatment commences there is preliminary treatment of the waste where a grit chamber will be used to separate Mud and Silt from the sewage waste. The mud and silt are transferred to a landfill for environmentally safe disposal. The importance of pre-treatment is to remove substances that would spoil the system of clarifiers during primary treatment (Arundel, 2000).
Sewage is then primarily treated where they are settled in tanks that settles approximately 60% of solids in the waste stream. The tanks are referred to as clarifiers or sedimentation tanks which are used to eradicate very few toxic chemicals (Spearing, 2007). The waste is then transferred to the secondary treatment plant where oxygen is added to promote bacteria growth. Waste stays for long in this stage and results in removing (80-90) % human and solid waste, together with a larger chunk of toxic chemicals (Arundel, 2000).
Sodium hypochlorite is then added to the effluent to eliminate bacteria after which Sodium bisulphate is added to neutralise the hypochlorite through de-chlorinating the effluent (Spearing, 2007). The treated water is then discharged through outfall tunnels that lead to the Massachusetts bay that is 100 feet deep (Deer Island). Effluent passes through many perforated pipes each containing eight pores that would ensure that effluent is introduced rapidly and completely mixed with the surrounding water. All the processes are effectively scrutinized to ensure environment protection. The sludge by-product is digested and transported through the inter Island Tunnel of MWRA for fertilizer manufacture at Pelletilizing factory in Fore River.
References
Arundel, J. (2000). Sewage and Industrial Effluent Treatment (2nd ed.). Oxford: Blackwell Publishers. Pp. 5-149
Spearing, M. (2007). Basic Principles of Waste Water Treatment (Vol. 2). New Delhi: IWA Publishers. Pp. 125-140
The Deer Island Sewage Treatment Plant website at:
<http://www.mwra.com/03sewer/html/sewditp.htm>
