Early this year a backyard reservoir was constructed in Carapongo, Lima with the aim to improve human health, the natural environment and household income. Based on previous indications of contamination of the Rimac River, one of three sources of irrigation water for Lima agriculture, a project has undertaken an evaluation of irrigation water quality, to determine the presence of heavy metals, bacteria and parasites. Whilst heavy metals were not found to be a major problem, the water had high levels of e-coli bacteria and parasites, mainly coming from untreated domestic wastewater released into the river. The presence of these contaminants on vegetables indicates a potentially serious health problem for consumers.

As a consequence, Urban Harvest and partners, by using a participatory research approach, installed a small pilot backyard reservoir, 185 cubic meters, to test the feasibility of using aerobic y anaerobic treatment technologies on a small water reservoir to clean the contaminated river water. The process begins when water enters the reservoir and is left static for about ten days, during which time the bacteria are eliminated through aerobic chemical processes and parasites removed through sedimentation. As part of the pilot study water from the treatment process and untreated river water were compared as irrigation sources for vegetables. The results have shown that the reservoir has cleared 98 percent of bacteria and eliminated all parasites from the river water. When radish and lettuce where tested for contaminants, those planted in treated water had up to 97 percent less bacteria (well below permitted limits) while the parasites were practically absent in both crops (see table 2).  

The loss of productive farmland occupied by the reservoir results in a net loss of about S/.500 ($150) to the farmer. To explore alternative economic opportunities, fish (tilapia) production in the reservoir is being assessed. Projections of potential net earnings from sale of fingerlings and mature fish is S/.1750 ($520). In addition to this benefit, irrigation with reservoir water appears to have had a beneficial effect both on rate of emergence and growth and on the uniformity of the crop, with higher percentages of marketable products available sooner than with use of river water. This difference is still under evaluation (see table 3).  The small reservoir has been shown to have multiple benefits, and is replicable in the area.  Working with local producers, Urban Harvest is therefore in the process of implementing other reservoirs with the capacity of irrigating up to 70 hectares with clean water.

Table 1.

Bacteria evaluation CF/100ml

According to the General Water Law, a maximum level of bacteria in CF/100ml, for irrigation water at 1000CF/100ml

Table 2.

Parasite evaluation parasites/ 100g

Table 3.

Economic comparison of crops irrigated with reservoir and river water

Both areas of cultivation 472.3 m 2

Significant difference, p<0.05