Best Practices Improvements: Nitrogen Pretreatment Performance of Land Based Wastewater Treatment Systems

Land based wastewater treatment systems can offer an efficient and cost effective wastewater treatment solution for communities and institutions that do not have access to a centralized wastewater treatment facility. Effluent from land based treatment systems is discharged directly into the ground rather than being sent to surface water, which minimizes negative impacts on freshwater ecosystems and also provides the benefit of recharge to local groundwater aquifers. The effluent discharged from these facilities must be monitored closely particularly in regards to effluent total nitrogen, to protect the quality of groundwater in areas where it's used as a source of drinking water. Unfortunately, many land based systems in Minnesota are currently having difficulties meeting effluent (i.e., end of pipe) total nitrogen permit discharge limits. With this in mind Natural Systems Utilities, (NSU) in conjunction with the Minnesota Pollution Control Agency,(MPCA) completed an evaluation of ten land based wastewater treatment systems with nitrogen pretreatment devices and effluent total nitrogen limits. The goals were to evaluate facility design, current operations and management practices, and explore ways to increase nitrogen removal efficiencies by implementing operational adjustments to the existing pretreatment and denitrification modules. The study was completed by first identifying suitable land based wastewater pretreatment systems based on criteria including current operations and flow status, technology type, and proximity to the Twin Cities Metro (TCM) area. Next, background data (e.g., facility design, existing performance data, etc.) was evaluated for each facility, and preliminary field and laboratory samples were collected to determine baseline treatment levels. Operational adjustments were then executed at each facility, and a second round of sampling was completed to determine the effectiveness of the adjustments. Data from both rounds of sampling was then compiled and used to determine which operational adjustments resulted in successfully lowering effluent total nitrogen concentrations (Appendix B-Table 2). The operational adjustments increased nitrogen removal efficiencies at six of the ten facilities, bringing two of the eight previously non-compliant facilities into compliance. This study produced a decrease in total nitrogen loading among all facilities of 1,544-lbs/yr, and additional monitoring over a longer period of time would likely yield further decreases in nitrogen loading to local groundwater aquifers. The results of this study illustrate a variety of operation and maintenance (O&M) issues that can lead to permit non-compliance, as well as some improvements that can be employed to mitigate compliance issues. Optimizing O&M is best accomplished with sufficient knowledge and resources. Land based wastewater treatment systems permit compliance is directly related to an experienced operator who understands the fundamentals of facility design and construction, follows best management practices (BMPs) for all O&M tasks, can identify how operational adjustments will impact treatment, and has an adequate budget to properly manage the system. Investing in education specific to land based wastewater pretreatment systems can be a valuable part of the development of knowledgeable operators who can quickly identify and resolve compliance issues. In terms of resource needs, it is recommended that operators complete at least one visit per week (two to four hours) to their treatment facility for O&M work. Optimizing O&M can also extend the overall life of a facility, but most importantly will help keep land based wastewater pretreatment systems in compliance through financially and environmentally responsible management practices.