Sources and Destinations of Wastewater
Wastewater comes from homes, and includes human and household wastes from toilets, sinks, baths and drains. It also comes from large and small industries, schools and business. Wastewater from these sources is collected in the sanitary sewer collection system and conveyed to the wastewater treatment plant for treatment. Another form of wastewater is rain water collected in street storm drains during a rainstorm. This is not truly considered wastewater but rather stormwater. This water does not go to the wastewater treatment plant. It is collected in the network of storm sewers separate from the sanitary system. It is conveyed directly to the local rivers with no treatment. This is the reason communities and the EPA stress not dumping or accidentally spilling anything down a storm drain or into the street.
Why is it Necessary to Treat Wastewater
The City's wastewater system provides essential public health protection for the people of Moline and all people and animal life that use the water downstream of our community. The federal Clean Water Act requires all municipalities like the City of Moline to treat its wastewater. In Moline, treated wastewater is discharged to both the Mississippi and Rock Rivers. Treatment helps protect aquatic life as well as keep waters safe for drinking, fishing and recreational uses. Our wastewater treatment plants remove suspended and dissolved solids, organic matter and pollutants from the water. They also disinfect the wastewater to ensure that discharged water does not contain harmful pathogenic bacteria life.
How does a Wastewater Treatment Plant Work
Wastewater plants are basically a compact, concentrated, and accelerated version of the natural cleaning process found in nature. There are four basic steps to treating wastewater at most all municipal treatment plants.
- The first step, called primary treatment, removes about 90% of the settleable and floating solids and 60% of the organics from the wastewater. Primary treatment first involves screens to collect trash as water passes. Next water flow is slowed in a sedimentation tank to allow heavy solids to settle and lighter greases too rise where each can be removed.
- In the second step, called secondary treatment, 85 - 95% of organic pollutants are removed. To achieve this process, air is mixed with wastewater to supply oxygen, along with natural occurring bacteria and other microorganisms that are grown and maintained in the treatment plant system. These organisms consume harmful organic matter and convert it into settleable matter. Next a sedimentation tank slows the flow allowing the microorganisms and the settleable matter to separate from wastewater.
- The third step involves disinfecting the wastewater. This process kills the pathogenic bacteria that are harmful to people and the environment. In this step, chlorine is added to the wastewater in a controlled and monitored environment to disinfectant it before being discharged.
- The fourth step in the treatment process deals with the treatment and disposal of all of the sludge solid by-products created in the wastewater treatment process. The solids that were removed from both primary and secondary treatment are sent for further processing called stabilization in digestion tanks where they further break down by decomposition. When stabilized, these sludge bio-solids meet all local, state and federal regulations for utilization for beneficial land application reuse as fertilizer for farm land. Currently, Moline’s South Plant bio-solids are applied to farmland as fertilizer.
What are the Beneficial Products of Wastewater Treatment
Three main by-products are created by wastewater treatment: water, methane gas and bio-solids. In Moline, the water is put back into the rivers. Some areas use it to recharge their reservoirs or for irrigation. Methane gas is produced by the decomposition of bio-solids. The gas is presently used to heat process tanks at the treatment plant. It will eventually be utilized to produce a portion of the electrical and heating energy needs of the treatment plant. Bio-solids are used to fertilize crop land as they have very good natural fertilizer quality.