Special report to the Mirror by Lorraine Huchler.
Municipal wastewater is a slurry of human waste, soaps, grease and whatever else folks flush or send down the drain and into the sewer. The WWTP may also process some or all of the storm water collected from runoff of paved surfaces.
The heart of this treatment process is the “bio-reactor” – the tank where bacteria consume the contaminants of wastewater: nitrogen, ammonia, phosphorous. Creating the right water chemistry for bacteria to thrive is similar to managing the aquatic environment for a fish tank. The important water chemistry parameters include pH, nutrients like phosphorous, dissolved oxygen and temperature. The bacteria consume the soluble contaminants such as phosphorous, reproduce and die, creating “bio-mass” that must be removed. The figure below show a process very similar to the Cape Charles treatment plant.
Step 1: Solids Removal. Most plants use a series of screens remove large particles and other solid contaminants.
Step 2: Pretreatment. A slurry of water and suspended solids enter the first vessel. Operators adjust the chemistry of the wastewater; they may have to add nutrients like phosphoric or nitric acid if there is not enough “food” for the bacteria. Operators also add chemicals like alum to help settle the suspended solids (sludge). This is a continuous process, and some of the sludge is routed to a filter press for disposal.
Step 3: Aeration Zone. The bacteria are very “fussy” about the concentration of dissolved oxygen; operators adjust the rate that pumps bubble air through the tank. Operators carefully monitor several parameters to balance the population of healthy bacteria with the conditions: amount of food, temperature, time for bacteria to consume the food. Often the bio-mass floats on the surface of the tank, forcing the operators to use the “scum pump” to remove those suspended solids.
Step 4: Membrane Filtration. Environmental regulations strictly control the clarity of the water. Membrane filtration produces very clear water, but it cannot removed dissolved contaminants such as phosphorous. The suspended solids filtered out of the wastewater are also routed to the sludge press for disposal.
In order for the wastewater treatment plant to consistently meet their discharge limits for contaminants to avoid fines and other penalties, the plant must have a high level of mechanical reliability and redundancy, experienced operators and conformance to a strict operating discipline. Understanding the reasons for poor operation of a wastewater plant requires a review of the equipment, the skill and experience of the staff and the management of the plant.
o This plant is relatively new; however, that does not mean that it has sufficient mechanical reliability and redundancy, especially with respect to installed or shelf spare pumps, storage tanks, controls flexibility, automated chemical feed systems, and on-line analyzers. Insufficient maintenance budget for repairs, equipment replacement and spare components is often the greatest source of poor performance of wastewater treatment plants.
o Wastewater treatment is part science, part art. The state of Virginia licenses operators based on their technical knowledge; however, there is no substitute for experience and personal initiative in these plants. Operators have to understand the process and the chemistry of wastewater as well as the specific limitations of their plant.
o Management is responsible for creating an effective operating discipline that sets expectations for data-based decisions for control of the process, data-review for process improvement, troubleshooting off-spec conditions, anticipating changes in the process, maintaining equipment in good working order and documentation of the process.
So, when the Public Works manager reports that the WWTP discharge exceeded the phosphorous concentration of the permit, that means that the operators were not able to precisely feed or monitor the chemistry in the bio-reactor or that the plant lacked sufficient volume of storage tanks to give the bacteria time to consume the available phosphorous nutrients.
It’s tempting to focus exclusively on this phosphorous information to fix the problem, but the issue is probably more subtle and more complex with a solution that may require new procedures and/or new equipment. It’s likely that the plant operators are the best source of information regarding the contributing factors and the options to prevent future violations.