A low-temperature solar thermal system assisting a biological nitrogen treatment reservoir was designed and field-tested. A large tank whose temperature was maintained at about $25-30^{circ}C$ to enhance the performance of a biological nitrogen treatment process was heated by an array of flat plate solar collectors. Test results revealed that the overall collector efficiency was above 50% for the most cases tested. This high efficiency was possible owing to the relatively low collector temperature that can be traced back to the reservoir temperature. A substantial enhancement in nitrogen treatment was observed as a result of maintaining the reservoir temperature higher.