Technical Report No. 128
TREATMENT OF MILILANI STP EFFLUENT FOR DRIP IRRIGATION OF SUGARCANE
Edward Liu and Reginald Young
August 1980
ABSTRACT
The secondary effluent of the Mililani STP is intended for use in the drip irrigation of sugarcane. The relative success of this waste water reuse application is affected by the concentration of suspended solids in the sewage effluent. Chemical coagulation, dissolved air flotation, and granular media filtration were studied in the laboratory to determine the most cost-effective means of obtaining a sewage effluent with a suspended solids content similar to that of the water from Waiahole Ditch presently used in drip irrigation. Of the three methods tested, granular media filtration with anthracite coal-silica sand media at a flow rate of .00l 7 m^3/s/m^2 (2.5 gpm/ft^2) was a cost-effective method of producing a sewage effluent with suspended solids concentration in the range of that found in Waiahole Ditch water during dry weather. Model laboratory testing shows that application of this sewage effluent will not adversely affect the type of filter media used in drip irrigation. Preliminary analysis of the residual solids distribution found in the sewage effluent after this posttreatment shows that failure of the drip irrigation system due to excessive plugging should not be a problem. The estimated annual cost of this treatment system is $147,000 based on a 0.16-m^3 /s (3.6-mgd) flow, or $40,830/mgd.