In June (2012), we provided an overview for people who are not water-plant operators of how drinking water is prepared for public consumption [1]. Now we want to do part two and cover the basics of treating wastewater.
First, a review of why knowing this is important: An important function of the governing body of a water system is to oversee the work of its operator(s) and the overall maintenance of the system. This involves occasionally making big decisions, which could include hiring a new operator or allocating thousands of dollars to buy a new pump. All members of a board and non-operations staff of a water system should have a basic knowledge of what it takes to manage and operate a system so they can make more informed decisions about it and do their jobs better.
One way to be informed is to talk to your operator. Have him/her come to you at a board meeting. Or, even better, go to his/her place of work and take a tour of your system, which is more than just the treatment plant. Especially at a wastewater treatment plant, all of your senses will be engaged, and you will understand the complexity of the operations.
Another way to learn a bit is by getting some information on your own. RCAP has produced resources that explain the technical aspects of operating a wastewater system in simple ways that non-technical people can understand.
How is wastewater treated?
Both drinking water and wastewater treatment processes involve the use of many types of resources: human (skills, experience); natural; energy; knowledge of physical, biological and chemical processes; use of math; and staying current with state and federal rules and regulations. There is space here to say only a little bit about treating wastewater, and this overview will address only centralized systems where wastewater is collected from the community and treated together in one place. Decentralized systems (sometimes known as septic systems), where wastewater is collected and treated onsite, near a house or building, will not be addressed here because there are many ways to do it, and that is a subject worthy of its own discussion.
As you can imagine, treating wastewater is more complicated than treating water for drinking. Think through your daily routine and all of the stuff you put into your wastewater – soaps used at sinks and in the shower, liquid and solid human waste, toilet paper, water from doing the dishes and laundry, solids from the kitchen drain, etc. Cleaning water with all of this in it and making it suitable for the environment again takes a lot of work.
First, there is the collection system that gathers up all the wastewater and sends it to the plant. The collection system includes not only the pipes connected to your house, but also sampling stations, pump stations, pressure-relief valves, vacuum breakers, manholes and shutoff valves.
There are three main phases of wastewater treatment in a centralized system:
- removal of large and floatable material
- treatment of wastewater
- treatment of sludge
Treatment begins by physically removing large items from the water, using equipment like screens, in the headworks of the plant. These items are usually sent to a landfill. Primary treatment follows, which in most plants is a physical process (using gravity) to remove some suspended solids and organic matter from the wastewater. In large primary sedimentation basins, settleable solids and floatable materials separate from the wastewater and are removed from it.
Secondary treatment removes biodegradable organic matter, nutrients like nitrogen and phosphorus, and suspended solids from the wastewater. There are many different processes that can be used to accomplish this goal: aerobic or anaerobic digesters, oxidation, membrane filtration, chemical treatment and more. Your system may use more than one of these technologies.
Disinfection is considered part of secondary treatment of wastewater. This can be accomplished through chemical, ultraviolet light or ozone processes. The most common way to disinfect water at this stage is to use chlorine. Disinfection inactivates biological contaminants in the wastewater prior to discharge of the water back into the environment.
The sludge that is removed from primary and secondary treatment processes must also be treated prior to being disposed. As for the water, look carefully at what is discharged from the plant and consider all of the ways it has been treated – physically, biologically and chemically – and you will be amazed at the clean, clear water that is produced. For the past 40 years, the Clean Water Act has regulated these processes to protect the environment and the public’s health. The act, along with other federal and state regulations, ensures that we can use this water again. After all, the wastewater that is treated will be used again by the planet and eventually our bodies.
Where to learn more
[2]RCAP has produced two publications for members of utility governing bodies:
- A Drop of Knowledge: The Non-operator’s Guide to Wastewater Systems [2]
- A Drop of Knowledge: The Non-operator’s Guide to Drinking Water Systems [3]
These guides provide basic explanations of the treatment processes in plain, everyday language for non-technical audiences. Included are many diagrams, illustrations and photos that explain the treatment steps.
The guides are available in print from RCAP staff in the field or as PDF, which you can view and print yourself on your own computer at www.rcap.org/commpubs#dok [4].
[5]There is also a multimedia companion [5] to the publications listed above. An animated diagram shows the drinking water and wastewater treatment processes, and video segments explain some of the steps and activities that an operator oversees in each part.