Investigations On Methods And Mechanism For Removal Of Nitrogen & Phosphorus In Wastewater By Immobilized Algae

Via a series of experiments on algae growth characteristics and removal of nitrogen and phosphorus, together with analysis of the experimental results, proper algae species and reasonable methods of immobilization is presented in this dissertation.During the investigations, the author found that the micro-structure in the carrier of consists of complex network channels. The algae cells are attached to the surface of algin. This kind of network structure is very helpful for the absorption of nutrition with nitrogen and phosphorus by the algae cells. The removal efficiency of nitrogen and phosphorus in wastewater was directly proportional to the holding capacity of algae cells in alginate balls. With high inoculating density of the immobilized Chlorella, the experiments got removal efficiency of 91.2% and 100% for NH₄⁺-N and PO₄^3--P respectively, in 3 days. After reharding, the maximum holding capacity was almost doubled than that of the system without reharding. The method of immobilization with reharding is effective to solve the problem that the balls loose their integrity and the algaes cells leach easily in the wastewater with phosphate. Therefore, the characteristics of immobilized balls and the removal efficiency for NH₄⁺-N and PO₄^3--P were greatly improved with reharding system. The static experiment of Lyngbya limnetica. attached system showed that the best removal efficiency of NH₄⁺-N,PO₄^3--P was achieved by conditions of 10:1 proportion of N/P, middling density of attaching material, 2 days hungry and biomass at 1.0g/L. According to the dynamic experiments, Lyngbya limnetica and Oscillatoria amoena Gom attached system got 85.9%, 68.5% removal efficiency for NH₄⁺-N and PO₄^3--P respectively in HRT 3 days. The dissolved oxygen in the effluent was increased obviously, and the water quality was greatly improved. All these show that immobilized algae have the potential application feasibility for the removal of nitrogen and phosphorus.The major manners of removal for nitrogen and phosphorus include: adsorption by algin balls at the starting phase; transmission of nitrogen and phosphorus; assimilation by Chlorella cells; gas stripping of NH₃ and phosphate precipitation caused by rising in pH. Immobilized Chlorella has unequal capability to assimilate various nitrogen resources. The sequence of absorption priority of different nitrogen by algae is NH₄⁺-N , urea, NO₃^- -N and NO₂^- -N. Within the same condition, the experiments of different phosphorus uptake by immobilized Chlorella showed that the order ofdifferent phosphorus uptake by Chlorella is ortho-phosphate, metaphosphate, polyphosphate, organic phosphate.Compared with attached algae system, embedded algae system has higher nutrient removal efficiency, and ingathering of algae is easier. However, attached algae system is easier to operation in engineering practice than the embedded one.Preliminary mechanism research on removal of nitrogen and phosphorus by immobilized algae is also presented in the dissertation.