Technical Report No. 62
First Progress Report for August 1971 to July 1972
Project Principal Investigator – L.Stephen Lau Co-Investigators: Soil and Irrigation Studies – Paul C. Ekern, Virology Studies – Philip Loh, Water Quality Analysis – Reginald H.F. Young, Overall Data Studies – Gordon L. Dugan – November 1972
An investigation of recycling sewage effluent by irrigation under Hawaiian conditions is being conducted in pilot field studies near Mililani Town in central Oahu under the sponsorship of the Board of Water Supply and the Division of Sewers, City and County of Honolulu. The primary objective of the project is to determine the feasibility of wastewater application to the soil and its probable effects on the quality of ground water in terms of dissolved materials and viruses. Corollary objectives are to ascertain its effects on sugarcane yield and grass lands. The studies began in-September 1971 with the construction of a five-foot deep hydraulic lysimeter in a grassed area on the grounds of the Mililani sewage treatment plant. The upper surface of the lysimeter was at ground level. Soil within the lysimeter was repacked to the approximate original density. In an adjacent site, a number of two-foot square pans were placed at various depths down to five feet in undisturbed soil adjacent to an access pit. The lysimeter and pan areas were sprinkler-irrigated with secondary sewage effluent from the Mililani sewage treatment plant on a regular schedule. Five furrows of maturing sugarcane in a nearby field were also irrigated with the secondary effluent while the adjoining furrows continued to receive regular irrigation water. Numerous point water samplers were positioned in both furrows and ridges of the sugarcane field at depths to 33 inches. The soil within the test sites of both the grass and sugarcane areas is of the oxisol Lahaina series, the general type on which approximately 90 percent of Hawaii’s irrigated sugarcane is grown. Raw sewage, secondary effluent, and leachate from the soils were assayed for various physical, chemical, sanitary, and microbiological quality parameters. Analyses for pesticides and heavy metals were also occasionally performed. A virus laboratory, the first of its kind in Hawaii, was established at the University to serve the project and to assist in training the personnel of the Board of Water Supply. Soil samples were analyzed periodically for changes in their mineral composition. Consumptive use of water was determined by use of the hydraulic lysimeter. Operational analyses showed that the Mililani sewage treatment plant, which employs the activated sludge process, is capable of removing a high percentage of the biodegradable substances and suspended solids as well as a surprisingly high percentage of nitrogen, a nutrient ordinary secondary treatment plants are not designed to remove. The sewage effluent was void of detectable mercury (less than 0.3 parts per billion) and the cadmium and lead were less than the Public Health Service drinking water standards. Pesticides were found to be less than 1 ppb. With the possible exception of the sodium percentage, the effluent appears to be of good quality for agricultural irrigation use. The soil at Mililani appears to be very effective in removing BOD5, TOC, nitrogen, phosphorus, potassium, boron, and coliforms as evidenced by analyses of the leachate passing through the five-foot deep lysimeter. Nitrogen, however, was not effectively removed until after five months of operation, whereas, the other constituents exhibited fairly rapid attenuation. There appeared to be evidence of a base exchange or similar phenomenon in the soil at least between sodium and calcium in Oahu Sugar Company sugarcane field No. 240. Similar results were also found for the leachate collected by the point samplers in the sugarcane fields. Virus cultures were obtained from the sewage effluent before chlorination and identification is presently (August 1972) proceeding. To date, data have been limited to those collected over a period ranging from several months in some instances to only one or two months in others. Thus, it is too early to formulate any definitive conclusions or to predict the concentrations of the various constituents which might be expected, especially since the full seasonal effects have not been determined.