WRRCTR No.170 Wastewater Irrigation for Alfalfa, Guinea Grass, and Papaya Production in Hawai'i

Abstract

A preliminary investigation was undertaken to determine the effectiveness of using treated municipal wastewater for irrigation and nutrient stripping by three agricultural crops. Alfalfa and guinea grass were chosen because local production could reduce the large amounts of alfalfa cubes and hay imported for the dairy and cattle industry. Papaya was selected because it is a developing export crop with an established marketing infrastructure. Health hazards were not a factor in this study. Forage crops are consumed by animals before reaching the human food chain. Papayas are harvested 5 to 10 ft above the ground (where drip irrigation lines were located), with no direct contact by the irrigated wastewater. Alfalfa produced 16.6 tons/acre/yr dry wt or 85 tons/acre/yr wet wt; guinea grass yielded 21.0 tons/acre/yr dry wt or 126 tons/acre/yr wet wt. Guinea grass contained 1.5 times more water than alfalfa, and although dry wt production was higher, its crude protein content was lower, amounting to 1.96 tons/acre/yr compared with 3.49 tons/acre/yr for alfalfa. Both forage crops stripped N from the effluent but guinea grass was more efficient than alfalfa. Nitrate levels of the guinea grass percolate were below the drinking water limit of 10 mg/l as NO3-N after the first harvest, while alfalfa gradually increased its stripping ability and exceeded the limit after the fifth harvest. Difficulty was encountered in obtaining a viable crop of papaya. Of the transplanted seedlings, only 25% survived and became established. Thus, female papaya plants were not culled and fruit production rate was measured for all the plants. Extrapolation of the total yield of 122,000 lb/acre/yr, of which 30% was marketable, indicated papaya production amounted to 36,000 lb/acre/yr which is comparable to commercial production in Kapoho, Hawai'i.