Treatment of reject water

Sludge from primary clarifiers and final sedimentation (i.e. excess sludge) are stabilized in an anaerobic digester, and dewatered thereafter. The water flows from sludge dewatering processes (i.e. reject water or sludge liquor) on municipal wastewater treatment plants (WWTPs) are a few liters per person per day. Therefore, the hydraulic influence on the plant is not important. In contrast, the ammonium concentration in the sludge liquor can reach 2500 mg/L. Thus, the rejected nitrogen load can account for up to 25% of the nitrogen load in the raw sewage. Due to the considerable nitrogen load, reject water from dewatering of sludge could impose on the wastewater treatment process. Being returned to the inlet of a wastewater treatment plant the influent loading of the plant is significantly increased causing occoasional overloading situations. Thus, separate treatment alternative is recommended as an optional solution to the problem.Various researches works into finding innovative ways that takes advantage of ammonium rich composition of reject water, and has gained prominence in the wastewater industry over the last two decades. Currently new heights have been attained through the nitritation/denitritation step especillay for ammonium removal. In addition, there are a number of methods for treating phosphorus in reject water namely; magnesium-ammonium-phosphorus (MAP) or struvite precipitation, hydroxyapatite (HAP) precipitation and natural aging of phosphorus. However, progress at phosphorus removal has largely remained at the experimental stage, in spite of its considerable composition in reject water. Therefore, the aim of this thesis is to try to identify theoretically all the methods that have evolved over the years at treating reject water from dewatering sludge and investigate the possible strategies to handle the rejection problem, and processes for separate treatment of the sludge liquor. In this framework, characterization of the reject water taken from two different municipal WWTPs is carried out as first. Characterizaion studies were assisted to realize that important parameters such as pH, nitrogen, phosphorus, suspended solids, coliform and heavy metals for treability studies and use of reject water for irrigation purpose. Afterwards, treability studies were made and struvite and hydroxyapatite precipitation methods were carried out for treatment purpose. Treability studies stated that %25-41 of phosphate phosphorus and %28-46 of amonium nitrogen and %76-100 magnesium were treated by struvite precipitation at optimum pH (8-9), mixing time (4 hours), perlit dose (5, 15 and 20 mg/l) and Mg-P ratio (1:1, 1.1:1). On the other hand, higher phosphate phosphorus removals were achived by hydroxyapatite precipitation. In the hydroxyapatite precipitation studies, phosphate phosphorus treatment ratio was around %92 at optimum pH (9), 2 hours mixing time and 20 g gypsium/L were selected as optimal dose.