Recovery of phosphorus and other minerals from greenhouse wastewater generated during soilless tomato cultivation by means of alkalizing agents

The research was aimed at determining the possibility of recovering part of nutrients by precipitation from greenhouse wastewater (GW) from soilless tomato cultivation. Analyses included such elements as: P, S, N, Cl, Ca, Mg, K, Mo, Mn, Fe, Zn, Cu, and B. Three alkalizing agents were tested in a pH range of 6.5-12.0: Ca(OH)₂, KOH, and NH₄OH, which simultaneously enrich greenhouse wastewater in calcium, potassium, and nitrogen. It was determined what dose of the alkalizing agent should be used, how the composition of the treated GW will change, how much and what kind of sludge will be formed, what will be the stability and technical possibility of sediment separation, and whether the type of alkalizing agent affects the course of the process. Precipitation triggered by the alkalizing agents proved to be an effective method for the recovery of phosphorus, calcium, magnesium, manganese, and boron, while it turned out ineffective in the case of the other elements tested, including nitrogen and potassium. Phosphorus recovery depended mainly on GW pH and forms of phosphate ions corresponding to this pH, and not on the alkalizing agent type. The pH value adjustment to pH = 9 for KOH and NH₄OH and to pH = 9.5 for Ca(OH)₂ ensured <99 % phosphorus recovery, which corresponded to P concentration in GW below 1 mgP/L and to the applied Ca(OH)₂, KOH, and NH₄OH doses of 0.20 g/L, 0.28 g/L, and 0.08 g/L, respectively. The highest P contents in the sludge were determined at pH = 7 and reached 18.0 %, 16.8 %, and 16.3 % in the experimental series with Ca(OH)₂, KOH, and NH₄OH, respectively. The sludge volume index increase along with pH increase up to pH = 10.5 for KOH and to pH = 11 for Ca(OH)₂ and NH₄OH.