Costs to operate membrane bioreactors increase with energy and carbon source prices.
Solution: Process promotes growth of microorganism that removes nitrogen and phosphorus with less oxygen and carbon addition.
Removing pollutants in wastewater requires intensive energy inputs and added chemicals, resulting in high costs. With increasing requirements for water resource recovery facilities (WRRFs) to discharge high-quality effluent, many search for treatment solutions to achieve the same level of treatment with lower energy and chemical inputs, according to Mark LeChevallier, director of Innovation and Environmental Stewardship at American Water (Voorhees, N.J.).
Mapleton and Jefferson Peaks WRRFs in New Jersey use membrane bioreactors (MBRs) to treat domestic wastewater generated from nearby communities. The facilities discharge high-quality effluent to groundwater. Because of increasing energy and carbon source costs, these WRRFs sought alternate solutions to reduce costs while continuing to produce high-quality treated effluent, LeChevallier said.
In traditional wastewater treatment systems, approximately half of the total energy consumed is used to maintain the proper dissolved-oxygen concentration in the aeration tank to remove organic matter and ammonia. Aeration energy intensity becomes higher when using membrane technology due to the cleaning requirements to prevent fouling on the membrane surface. And supplemental carbon sources, such as methanol, may be added to an anoxic tank to enhance nitrogen removal, LeChevallier said.
As an alternative, the two WRRFs installed the NPXpress wastewater treatment and nutrient removal process. This process, developed by scientists and engineers in the Innovation and Environmental Stewardship Department of American Water, is designed to treat municipal wastewater in full-scale MBR facilities to produce high-quality effluent while using less oxygen and added carbon.
The process promotes growth of a certain microorganism that removes nitrogen and phosphorus. The microorganisms require less oxygen and carbon than conventional bacteria in traditional activated sludge systems used to convert ammonia–nitrogen to nitrogen gas, LeChevallier said.
Reaping unexpected rewards
Since implementing the technology at the two facilities, aeration energy consumption has dropped up to 50%, and supplemental carbon addition has been eliminated. At Mapleton, overall energy consumption dropped by approximately 30%, and energy intensity decreased by approximately 30%. At Jefferson Peaks, overall energy consumption dropped by approximately 50%, for a cost savings of approximately 54%. The additional saving at Jefferson Peaks was attributed to the fewer demand charges from the electrical utility. Eliminating supplemental carbon at these locations translates to further savings on chemicals and the related operation and maintenance, LeChevallier explained.
“During and after the implementation of the technology, the plant was able to consistently achieve good performance in terms of total nitrogen removal, and there were no negative effects observed,” LeChevallier said. “In addition, due to the implementation of the technology, enhanced biological phosphorus removal was observed as a side benefit for nutrient removal.”
The simple payback period, calculated by comparing the capital investment and the savings at the two full-scale facilities, was 3.1 and 4.6 months, respectively, for the Mapleton and Jefferson Peak facilities, LeChevallier said.
The NPXpress process received a patent in September 2011 and is being installed at six other American Water WRRFs.
“The NPXpress technology has demonstrated promising capabilities to achieve wastewater treatment and nutrient removal in a sustainable way and provided opportunities for wastewater utilities to design, build, and operate an energy-neutral wastewater treatment plant,” LeChevallier said.