Integrated Wastewater Treatment and Resource Recovery for A Circular Economy Model in Decentralized Communities: A Case Study of Military Battalion in Vietnam

Abstract

Decentralized communities, including remote military battalions, often lack centralized wastewater treatment infrastructure, resulting in pollution from livestock wastewater. This study developed a low-cost circular economy system for a military battalion in Thai Nguyen Province, combining mechanical solid‒liquid separation, a combined anaerobic‒aerobic biological reactor, and a subsurface-flow constructed wetland planted with elephant grass (Pennisetum purpureum). The system was operated at 12–15 m³ day^-1 to treat piggery wastewater while nutrients were recovered for onsite agricultural reuse. The system was monitored over a 10-month period, comprising a 2-month startup phase and 8 months of stable operation. The system achieved removal efficiencies of 84.5% for BOD₅, 84.7% for COD, 53.3% for TSS, and 91.7% for coliform, consistently meeting Vietnamese discharge standards (QCVN 62-MT:2016/BTNMT). The effluent retained nutrient levels (approximately 116 mg TN L^-1 and 12.5 mg TP L^-1), enabling its direct use as a liquid fertilizer. When combined with compost produced from separated solids, the recycled nutrient system supported Brassica juncea (mustard greens) growth under the four nutrient management treatments. Compared with conventional chemical fertilization (36.46 t ha^-1), the treatment using recycled effluent plus high-dose compost (15 t·ha-1) resulted in a 23.6% higher marketable yield (45.05 t ha^-1). Nitrate, heavy metal, and E. coli concentrations in harvested vegetables remained within Vietnamese food safety standards. The circular system generated a net economic benefit of approximately 31.67 million VND ha^-1 crop^-1 cycle^-1 through the elimination of chemical fertilizers and increased crop revenue when it was operating at low energy intensity (0.25 kWh m^-3). This hybrid nature-based system demonstrated promising technical performance and agronomic benefits under the pilot conditions studied. However, longer-term monitoring and comprehensive nutrient balance analyses are recommended to fully validate system sustainability and the potential for fertilizer substitution. The approach offers a practical model for decentralized military and rural communities in tropical regions.