Y. Azov

Operation of high-rate oxidation ponds: theory and experiments

Abstract Optimal operation of high rate oxidation ponds depends mainly on the climatic conditions of the ponds location. The major factors affecting pond performance are solar radiation and water temperature, the former determines mainly maximal algal productivity while the latter determines optimal retention time operation. Pond dimensions, namely area and depth, are the only true independent variables which can be manipulated to produce optimal retention time in year-round operation. In the tropical regions a constant retention time is most adequate for optimal operation as water temperature is fairly constant year-round and a constant area and depth strategy can be adopted. In moderate climates seasonally variable retention time is preferred and it was found to be best economically achieved by varying pond depth at constant area. Another plausible strategy with possible economic advantages is a combination of dual function ponds operating solely for wastewater treatment during winter while some are converted into fish-rearing ponds during summer.

Removal of trace metals from wastewater during long-term storage in seasonal reservoirs

The removal of five metals (Cu, Zn, Cr, Pb, Al) was studied in two reservoirs in series used for the seasonal storage of wastewater effluents for irrigation. The evaluation was made by two methods: (1) an anual budget which includes inputs and outputs and. (2) sediment traps. The concentrations of metals were reduced between 20 and 75%, to the base level found in unpolluted groundwater in the region. The amount of Pb was reduced in 5%, Cu in 10%, Al in 30%, Cr in 50%, and Zn in 90%. Sedimentation has an irregular pattern due to the effect of wind induced longshore and rip-currents. The release of bottom sediments in the outflow means a direct release of trace metals and other settling pollutants and clogging particles. It is recommended to take the effluents for irrigation from the uppermost water layer, to avoid strong outflow rates which may drag out part of the sediments by hydraulic turbulence, and to locate the outlet away from the dominant wind axis. The main tools to improve the removal of trace metals in seasonal reservoirs are not the control of the age distribution of the effluents and/or the loading of the reservoirs, but the proper location, design and flow rate of the outlet.

Effect of effluent addition to a freshwater reservoir on the filter clogging capacity of irrigation water

Abstract This paper deals with the effect of the addition of effluents to a shallow freshwater reservoir used for irrigation, on the clogging capacity of the water. The clogging capacity was estimated with an instrument that measures the increasing differential pressure needed to maintain a constant flow of water through a 30 μm screen filter. The dependence of the clogging capacity on plankton composition and suspended solids concentration was studied by means of multiple regression analysis, on four different arrangements of the matrix of data. The addition of effluents raised the clogging capacity sharply and quickly. The phenomenon was primarily due to an increase in the number of large algae and zooplankton species. These large species partially substituted for the small ones which were dominant before the entry of effluents. The concentration of suspended solids also affected the clogging capacity, but to a much lower extent. A relationship between the clogging capacity and the total number of plankton organisms was not found. The addition of plankton filter-feeding fish to the reservoir is recommended.

Plankton community changes due to the addition of treated effluents to a freshwater reservoir used for drip irrigation

Abstract The present paper defines the structure and dynamics of the planktonic populations before and after the addition of reclaimed effluents to an operational irrigation reservoir. The analysis of the effects of the inflow of the effluents on the parameters that characterize the planktonic populations may serve to determine the means that have to be taken to improve the quality of the water used for drip irrigation. The planktonic communities in the reservoir were defined by factor analysis. The input of effluents led to a change from phyto- and zooplankton communities dominated by small species, to communities dominated by large species (greater than 200 μm). This change was characterized by an increase in biomass without an increase in the total number of algae. The dominating algae were species of Cyanophyta which responded to the input of N compounds rather than to the increase in phosphate concentration. Laboratory enrichment experiments support the idea that the limiting factor was the N compounds.