Document Type


Publication Date

January 1982


The effectiveness of a sloped rock-grass filtration system in treating municipal wastewater was tested on a 24 m by 36 m (80 ft by 120 ft) slope on a 2.5 percent grade sown with a mixture of rye grass, fescue grass, and bluegrass. The field was divided into six plots, each approximately 3.5 m wide and 36 m long. Three of the plots (slope-rock) were constructed with 9 m of gravel, 7.6 cm deep, on the upper reaches of the slope. Raw (screened, degritted) municipal wastewater from Hyrum, Utah, was applied to the slope-rock sections at application rates of 13 and 20 cm/wk the first year of operation (June through October 1979) and 23, 41, and 51 cm/wk the second year (June through October 1980). The other three plots were constructed as conventional overland flow slopes. Wastewater was applied at rates of 13 and 20 cm/wk the first year and 23, 41, adn 57 cm/wk the second year. The gravel layer increased infiltration and, therefore, decreased the amount of wastewater effluent recovered. The gravel also increased wastewater detention times on the treatment slopes. In general, the slope-rock sections acheived higher mass removal associated with greater water losses. However, the gravel layer had no statistically significant effect, at the 95 percent confidence level, on the concentration of pollutants. On a concentation basis, BOD5 removal for the test sections were 87 to 93 percent. BOD5 effluent averages ranges from 6 to 12 mg/1. Mean effluent suspended solids ranged from 5 to 9 mg/1. Even at the highest hydraulic laoding rate (57 cm/wk), effluent quality met the 1985 State of Utah effluent limits. Total phospohorus reductions were only 20 to 33 percent. Orthophosphate concentrations increased on all slopes. Ammonia removals, 69 to 93 percent, were achieved at the lower loadings (13 cm/wk, 20 cm/wk and 23 cm/wk). The highest loading (57 cm/wk) exhibited 33 to 43 percent removal. Nitrification of ammonia occurred on all the slopes. Fecal coliforms were reduced by as much as 99 percent on some of the slopes, but effluent fecal coliforms were not reduced below 10^4 colonies/100 ml. Harvesting temporarily decreased system performance. Effluent BOD5 and suspended solids concentrations, however, still did not violate effluent discharge limits (i.e., 15 mg/1 BOD5, and 10 mg/1 SS, 30 day average) for the State of Utah.