Tailored water treatment in distributed wastewater reclamation: investigation of biological stability, and energy and nutrient recovery

Abstract

Distributed non-potable water reclamation (DNWR) is emerging as a new model that couples local water resources with local users, tailoring local water treatment to the needs of the end-users. Yet, there are barriers that must be addressed—DNWR plants are likely to be near homes and businesses, and the application of the reclaimed water for uses like irrigation or ground/surface water augmentation has the potential to directly impact local residents and the environment. Because of the potential impacts to human health and utilities’ risk-aversion, communities are slow to adopt new technologies for reclamation. Thus, novel operation and applications of established technologies are needed to advance DNWR. An additional challenge for DNWR facilities is the higher energy use per unit of water treated. Studies show that the energy required for wastewater treatment increases by 65% when flows decrease from 38 to 3.8 million liters per day (ML d–1) (10 Mgal d–1 to 1 Mgal d–1). The low energy efficiency is exacerbated by lack of technologies for energy recovery from wastewater—energy recovery from solids is infeasible for facilities treating less than 19 ML day–1 (5 Mgal d–1). Sequencing batch bioreactor (SBR) is an activated sludge technology recognized as a reliable and robust biological process, well-suited for small facilities. An SBR system was used in this study to investigate unique tailored water treatment options: enhancing the primary clarification prior to SBR treatment to produce nutrient-rich water for irrigation (fertigation) and improve energy recovery, and changing the operating condition in the SBR to reduce the energy used by aeration blowers. A bench scale SBR coupled with conventional primary clarification (CPC) was operated under hypoaerobic conditions (low dissolved oxygen (DO) concentration) during the aeration. The blower run time and removal efficiency of pollutants were monitored and compared to baseline operation (high DO concentration). Subsequently, a demo-scale SBR (7,000 gal d–1) coupled with membrane filtration was used to repeat the investigation and more accurately control DO concentrations and measure energy savings. The bench-scale equipment was also used to investigate enhanced primary clarification (EPC) coupled with SBR to produce fertigation water. Testing continued for 114 days during which time the blower run time and nutrient concentrations of the effluent were monitored. The long-term testing established the viability of treatment, and effluent concentrations were commensurate with target values. EPC and CPC results, in conjunction with modeling and analysis of solids from a wide variety of wastewater treatment systems were used to investigate enhanced energy recovery from wastewater at distributed wastewater treatment facilities. A model of air-blown gasification was developed and validated, and solids samples from wastewater facilities were collected and analyzed for thermal properties. The model calculated the energy production potential for solids produced by standard wastewater treatment (i.e., CPC-SBR), and for sludge produced by an EPC-SBR system. Air-blown gasification was economically feasible for plants with flows greater than 8 ML d–1 (~2 Mgal d–1), and although the source of the solids did not substantially change their energy content, the reduction in the mass of solids produced by EPC-SBR treatment reduced the overall plant energy production potential. Overall, this research focused on employing novel applications of existing commercially developed technologies to address the challenges of DNWR facilities. The results demonstrate that slight changes to operating conditions can reduce energy use and produced two different reuse waters, and the evaluation of air-blown gasification highlights a potential for energy recovery at DNWR plants.