The Secondary Sewage Treatment
The Primary and Secondary Sewage Treatment uses new technology to improve the cost and the performance of the sewage treatment used by the MWRA. The process is as follows:
1. Sewage is piped from communities to several head-works where bricks, logs and other large objects are screened out.
2. Pumps draw the sewage through deep-rock tunnels under the harbor to Deer Island.
3. Mud and sand settle in a tank called a grit chamber.
4. Later, this material, known as grit and screenings, is taken to a landfill for environmentally safe disposal.
The sewage then flows to primary settling tanks where up to 60% of the solids in the waste stream settle out as a mixture of sludge and water. This primary treatment removes very few toxic chemicals.
In the secondary treatment plant oxygen is added to the wastewater to speed up the growth of microorganisms. These microbes then consume the wastes and settle to the bottom of the secondary settling tanks. After secondary treatment, 80-90% of human waste and other solids have been removed. This process also removes a major amount of toxic chemicals.
There are two tunnels in the Massachusetts Bay that are used to release discharge 9.5 miles away from Deer Island. These tunnels are called Effluent Outfall Tunnel and the Inter-Island Tunnel
Sludge Into Fertilizer
In 1991, the process of turning sludge-to-fertilizer began. Sludge was no longer discharged into the Boston Harbor. Rotating, high-temperature dryers are used to kill all the unwanted elements in the sludge. The plant produces a small, hard granule that is approximately 60% organic matter.
The pellets contain several important nutrients such as, nitrogen, calcium, and sulfur. The nitrogen in the fertilizer is in an organic form, so it is able feed plants slowly and minimizes the risk of nitrate pollution.
The fertilizer is used in many ways. It is used for landscaping, gardening, and large-scale agriculture. The fertilizer also helps improve soils textures and moisture-holding capacity.