Phosphorus removal is a challenge undertaken by more and more utilities in the pursuit of cleaner effluent and protection for the environment. Phosphorus and other nutrients are tied to increased algae blooms, which can overload the ecosystem, causing depletion of oxygen and an overall negative effect to aquatic life and water quality. The United States Environmental Protection Agency (EPA) National Pollutant Discharge Elimination System (NPDES) monitors both the levels of phosphorus in the effluent and the seven-day low flow (7Q10) average of the receiving stream. The EPA is tightening standards on phosphorus discharge.
There are numerous methods to remove phosphorus from wastewater, ranging from membranes to coagulants, and this article deals with one particular wastewater treatment plant and their experiences.
Vernon Hills NCT Water Reclamation Facility
The Vernon Hills New Century Town (NCT) Water Reclamation Facility in Vernon Hills, Illinois, is part of Lake County Department of Public Works located northwest of Chicago. The plant utilizes a biological method for removing phosphorus using phosphorus-accumulating organisms (PAOs). The PAOs require a carbon food source to maintain a consistently sized colony to consume phosphorus. In some influent streams, there is adequate biological oxygen demand (BOD) to meet the nutrient levels needed to maintain this biological mass; however, BOD levels can change seasonally and are not a reliable source to ensure the colony is thriving. At the Vernon Hills plant, the BOD levels drop every summer, so it is necessary to augment the BOD with a carbon rich source to aid the PAOs in this process. A molasses-based product has proven to be a reliable way to help keep this biological mass healthy.
Of course, molasses is not the typical chemical/fluid that would be used at a water treatment plant and requires special consideration for both storage and pumping. Here are some specific characteristics of the molasses product:
- color—dark brown
- weight—11 pounds/gallon
- viscosity—10,000+ centipoise (increases at lower temperatures)
- 68% solids
Pumping this molasses product into the process has been a challenge for the plant staff.
Diaphragm pumps would not be effective due to the inability of the check balls to seat, rendering the pump ineffective. Diaphragm pumps also create limited suction lift, which would be problematic when the storage tank level drops. Several different models of flexible tubing-type peristaltic pumps were used with somewhat disappointing results. The suction lift of flexible tubing pumps is greatly reduced when the viscosity of the fluid pumped is high.
When the tank level dropped, it is possible that the tubing-style pumps could not lift the molasses to the pump for discharge. It became a struggle to achieve reasonable tubing life, which increased both the cost of replacements and maintenance man-hours.
In an effort to reduce the viscosity of the molasses, an experiment was conducted to see if dilution of the molasses with water would help eliminate some of these issues, but this required more labor and equipment, so the idea was deemed less than ideal.
One pump manufacturer recommended a peristaltic hose pump. These pumps are used in applications with highly viscous and gritty fluids. The peristaltic hose pump that was put into service uses a durable, reinforced, multilayer hose that allows for more reliable suction lift of high viscosity fluids and provides lower overall maintenance cost since the hose provides months of service before changing.