Masters Thesis

The Effect of Sewage Effluent on Soil Algae

Purpose of the Study: Land has been set aside to study the feasibility of double-cropping using waste water effluent from the Rohnert Park sewage treatment plant for irrigation. Experiments were conducted for one year to evaluate the growth of soil algae in the field irrigated with sewage effluent. An adjacent field that received no water during the summer, and a field that received only city well-water were used as a basis for comparison. Procedure: Soil samples were collected and analyzed from the three sites every month. Laboratory experiments included a quantitative determination of algal cells per cubic centimeter of soil and qualitative determinations of the composition of the algae present, seasonal succession of dominant populations, and nitrogen fixation capacities of the soils. Findings: Results of this study show that irrigation with sewage effluent in the summer greatly enhances algal growth in the soil throughout the year. The composition of the algae in the sewage-irrigated soil was comparable to that of the control sites. All three fields generally revealed the same seasonal succession patterns of algae. Nitrogen fixation studies demonstrated that sewage irrigation does not inhibit or enhance the amount of nitrogen fixed by the blue- green algae and bacteria. Conclusions: Irrigation with sewage effluent caused an increase in algal growth in soils of the experimental field, which contained substantially higher numbers of algae throughout the year. Although irrigation with effluent alters the number of algae in the soil, it does not affect the dominant populations of algae or alter the seasonal succession patterns. The nitrogenous compounds entering the soil with the effluent were thought to inhibit the nitrogen fixation mechanisms of certain soil organisms. In the present study a certain amount of nitrogen fixation was found to occur at all times in the soils irrigated with sewage effluent.

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