WRRCTR No. 94 Water Recycling of Sewage Effluent by Irrigation: A Field Study on Oahu
WRRCTR No. 94 Water Recycling of Sewage Effluent by Irrigation: A Field Study on Oahu
Lau, L. Stephen
Ekern, Paul C.
Loh, Philip C.S.
Young, Reginald H.F.
Dugan, Gordon L.
Water Resources Research Center, University of Hawaii at Manoa
The specific project objectives were to: (1) evaluate by field lysimeters and pilot plots and augment by laboratory studies the feasibility of utilizing water reclaimed from sewage for irrigation under Hawaiian conditions; (2) assess the probable effects of surface-applied reclaimed water on groundwater quality particularly in terms of potential viral transmission and long-term buildup of solids; (3) evaluate the effects of various water quality parameters on the soil, percolation, and vegetative growth when grassland or sugarcane is irrigated with sewage effluents; (4) explore any problem in sugarcane culture, either in technology or in crop quality that might be involved in the irrigation of sugarcane with water reclaimed from sewage. The central Oahu project site area is located near the Mililani Sewage Treatment Plant (STP) which, in 1975, received approximately 3217 m3/ day (0.85 mgd) of essentially domestic sewage from the nearby expanding Mililani Town development. The STP utilizes the Rapid Bloc activated sludge process (secondary treatment) that achieves a suspended solids and BOD5 removal rate that averages 90%. The location of the project site was chosen in part because the adjacent field soils are of the Oxisol order similar to that on which approximately 90% of the sugarcane cultivated under irrigated conditions on Oahu is grown. The general project site area receives an average annual rainfall of approximately 102 cm (40 in.), and is situated at an elevation approaching 152 m (500 ft). The research activities were grouped into three major areas: soils and irrigation, viral analysis, and water quality analysis. In general, the values of guideline chemical parameters for the Mililani STP effluent are below the maximum value for irrigation of sensitive crops. Pesticides and heavy metal concentrations were either below the level of concern or level of detectability. Nitrogen was given special emphasis for several reasons: its use as a major component of most fertilizers; its known adverse effect (lowered sugar yields) on matured sugarcane; its essential solubility in the nitrite and nitrate form; its relationship in concentrations above 10 mg/l as N to methomoglobinemia, the disease of infants; and its potential rote in the eutrophication of open bodies of water receiving excessive nitrogen loads. Commencing in August 1971, the project activities consisted of: the installation of field grass-sod, bare soil, and field lysimeters at the Mililani STP; coordinating laboratory facilities and analytical capabilities for determining the constituents in water, waste water, and soils; development of virus culturing and assaying techniques under field conditions, and studying the application of secondary effluent to maturing sugarcane in OSC Field No. 240, located approximately 3.2 km (2 miles) from the Mililani STP. The results of these studies helped establish procedures and guidelines for the principal focus of the project, the sequential application of sewage effluent, ditch water, and combinations thereof, to sugarcane in 30 test plots with uniform areas of 0.04 ha (0.1 acre) each in the newly planted (February 1973) OSC Field No. 246, located approximately 1.6 km (1 mile) from the Mililani STP. The test plots were divided into three basic irrigation schemes of ten plots each: A, B, and C. Plots "A" were scheduled to received only ditch water for the 2-yr growth cycle, "B" plots to receive secondary effluent for the first half of the growth cycle and ditch water thereafter, and "C" plots to have only effluent irrigation applications for the full growth cycle. Fifty ceramic point samplers were installed in representative "A", "B", and "C" plots at depths of 23 to 30 cm and 46 to 53 cm (9 to 12 in. and 18 to 21 in.). which resulted in the shallower points being positioned in the tillage zone and the deeper points being positioned approximately 15 cm (6 in.) below the tillage zone. Thus leachate collected by the shallower points represented liquid available to the sugarcane root zones whereas, leachate collected from the deeper points is assumed to be generally unavailable to the sugarcane and potentially may percolate to the groundwater table. Two 1.52-m (5 ft) deep field lysimeters were also installed in a furrow row adjacent to the test plot. The sugarcane growing on one lysimeter was irrigated with ditch water while sugarcane on the other lysimeter received secondary effluent. Sugarcane parameters were monitored periodically throughout the culture cycle. Field No. 246 was harvested in March 1975 and the associated laboratory analysis of the yields was completed and evaluated in April 1975. The Mililani STP secondary treated and chlorinated domestic and municipal sewage effluents containing insignificant amounts of toxic chemicals represent a generally usable irrigation supply for sugarcane and grasslands in central Oahu. Application of sewage effluent for the first year of a 2-yr cane crop cycle increased the sugar yield by about 6% compared with the control plots. However, when sewage effluent was applied for the entire 2-yr crop cycle, sugar yield was reduced by about 6% and the cane quality by about 16% even though the total cane yield increased by about 11%. There was no apparent evidence of significant surface clogging of the soil or of soil chemical properties impairment resulting from sewage effluent irrigation during the first full 2-yr sugarcane crop cycle. Under a no moisture stress condition, a 1-mgd supply is sufficient to irrigate 61 to 81 ha (150 to 200 acres) of sugarcane by the furrow method. The quality of percolate from the effluent-irrigated sugarcane-cultured soil was of acceptable concentration from the standpoint of groundwater quality protection: the only possible concern was for nitrogen which sporadically exceeded the 10 mg/l limit for drinking water during the first 6 to 7 months of cane growth. However, similar exceedance occurred in the ditch water-irrigated sugarcane plots and the plots irrigated with effluent during the first year and with ditch water during the second year. Furthermore, there was no major difference in the total quantity of nitrogen produced in the percolate among the three different treatments. Phosphorus, potassium, suspended solids, biochemical oxygen demand, total organic carbon, and boron were removed effectively from the applied effluent by means of irrigation; however, chloride in the percolate was essentially unaffected except for a transient increase during fertilization. Both total dissolved solids and chloride in the percolates met drinking water standards. Human enteric viruses have been shown to be present in the majority of effluent samples examined and hence, can be assumed to be present in the effluent applied to the irrigated field. However, the absence of these viruses in all sugarcane and grass percolates sampled over a 2-yr period, plus other project virus studies conducted, suggest strongly that the possibility of contaminating deep underground water sources is extremely remote. Survival of poliovirus was minimal in an open field area which was exposed to direct sunlight, high temperature, and dessication. In contrast, the viability of the virus was maintained for up to two months in a field of mature sugarcane where the virus was protected from the physical elements. Bermudagrass, with periodic cutting and harvesting, proved to be an excellent utilizer of sewage effluent applied nitrogen and, thus, excelled sugarcane from the standpoint of groundwater protection. Essentially no nitrogen was recovered from the percolate at the 1.52-m (5 ft) depth below the grassed surface, whereas nearly 25% of the total nitrogen applied from chemical fertilizers and sewage effluent was recovered at the same depth in sugarcane percolate. Up to 40.47 ha (100 acres) of grassland may be irrigated with 1 mil gal/day of effluent under a no moisture stress condition. However, it has been demonstrated that fallow or bare soil appears incapable of removing significant amounts of nitrogen from the applied effluent. Disinfected sewage effluent, similar in composition to that used in the Mililani study, may be used for irrigation of sugarcane in the first year followed by irrigation with surface water in the second year, however, when used for the entire 2-yr crop cycle without added treatment, poorer sugar yield will result. Establishing a virus monitoring and quality control program for the treated sewage effluent before application is an essential part of an irrigation recycling program. Furthermore, development of more effective methods of virus inactivation prior to recycling is highly recommended. Precautionary sanitation measures for field workers should be practiced. Further research on the use of effluent for irrigation sugarcane would be desirable, specifically: 1. Repeat test plot studies for a ratoon crop cycle to confirm the yield and to assess Long-term effects on the soil 2. Test with various dilutions of sewage effluent and with chemical ripeners to improve the yield 3. Investigate plugging of drip orifices in irrigation tubings in anticipation of extensive future use.
Water reuse -- Hawaii -- Oahu., Sewage irrigation -- Hawaii -- Oahu., Water quality -- Hawaii -- Oahu.
Lau LS, Ekern PC, Loh PCS, Young RHF, Burbank J, Nathan C, Dugan GL. 1975. Water recycling of sewage effluent by irrigation: a field study on Oahu. Honolulu (HI): Water Resources Research Center, University of Hawaii at Manoa. WRRC technical report, 94.
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