Agro-processing waste assessment and management in peri-urban Hanoi, Vietnam

A cluster of small- and medium-sized starch processors in peri-urban Hanoi, Vietnam generate income and improve the household economy by adding value to cassava and canna roots. Such seasonal and clustered processing activities tend to generate more waste than can be utilized or managed. A situation analysis showed that cassava and canna starch processing activities in three villages generated almost 1.45 million cubic meters of wastewater during the 1999-2000 processing season. The organic matter contained in the wastewater at processing points and in downstrearn ponds, exceeded critical values set by the government. Local residents perceived the wastewater as an environmental health hazard that requires attention. The potential of wastewater as a nutrient-rich irrigation source for selected crops was subsequently evaluated in one processing village in two trials conducted concurrently between March and June 2001. The trial results regarding crop performance showed that: (1) applying 80% wastewater resulted in the highest yield (p < 0.001), (2) applying wastewater during the first six weeks of the crop resulted in significantly higher yield (p < 0.001) than applying during the last six weeks, (3) applying wastewater once a week yielded better results than applying it every other week, and (4) applying unprocessed wastewater resulted in higher yields than applying settled wastewater (p < 0.001). The results also showed that the fecal coliform and worm eggs in the water, which mainly come from pig production, could offset these advantages. The levels of these microbiological contaminants could be manageable by taking proper precautions and planting standing crops such as rice. The results also showed the potential impact of wastewater on soils, the most marked of which was the increase in soil salinity. Despite the potential negative implications, applying root crop-processing wastewater for irrigation showed potential requiring further research to address the problem of contamination and soil salinity. Subsequent trials were designed to find ways to overcome the contamination problem and optimize production with wastewater. The trials conducted during March and June 2002 showed, with rice planted at 40 clumps/m(2): (1) when irrigated with wastewater twice (5.80 t/ha rice yield) or three times (5.90 t/ha rice yield) over the 12-week growing season, the yields are not significantly higher than the control (5.55 t/ha rice yield) which was irrigated with regular irrigation water; (2) when irrigated with wastewater four, five, or six times (6.60 t/ha rice yield) during the season, the yields are significantly higher, with yields increasing proportionally with the frequency of wastewater irrigation, and 3) the yields of rice irrigated five or six times with wastewater are comparable to the yields of fertilized rice irrigated with regular water. The implications of these results are that farmers may increase productivity while avoiding contamination if they avoid wading in the field during the four, five, or six weeks of wastewater irrigation. The other set of results showed that, when wastewater irrigation is fixed at six times, higher rice plant density leads to higher productivity. In other words, planting density of 35 clumps/m(2) results in significantly lower yield (5.94 t/ha rice yield) than at 40 (6.6 t/ha rice yield), while planting density at 60 leads to the highest productivity (7.28 t/ha rice yield). These results indicate that wastewater irrigation favors higher density rice production. The two sets of results suggest that yield can be significantly increased even if wastewater is applied only two or three times a season. during this short period of time farmers can stay out of the fields to avoid coliform contamination. (C) 2004 by The Haworth Press, Inc. All rights reserved.