Recycling of domestic wastewater treated byvertical-flow wetlands for irrigating crops

Due to water scarcity in many semi-arid countries, there is considerable interest in recycling various nutrient-rich wastewater streams, such as treated urban wastewater, for irrigation in the agricultural sector. The aim is therefore to assess if domestic wastewater treated by different sustainable wetland systems (some contaminated by diesel spills) can be successfully recycled to irrigate commercially grown crops such as Sweet Pepper (California Wonder; cultivar of Capsicum annuum Linnaeus Grossum Group) and Chilli (De Cayenne; Capsicum annuum (Linnaeus) Longum Group 'De Cayenne') grown either in compost or sand within a laboratory environment. The objectives were to assess the suitability of the irrigation water for long-term growth when using recycled wastewater, the impact of different treated wastewaters as a function of the wetland type, the impact of treated wastewater volume for irrigation, the suitability of different growth media for vegetable growth irrigated with treated wastewater, the effect of a diesel oil spill on the suitability of the recycled wastewater for vegetable irrigation, the economic return of various experimental systems in terms of marketable yields, the impact of differently treated wastewater on soil and fruit mineral and microbial contamination as a function of the wetland type as well as its operation and management, and the possibility of regenerating Capsicum annuum using the mother plant’s seed and irrigation with recycled wastewater treated by constructed wetlands to obtain a new cultivar adapted to urban wastewater. Vertical-flow constructed wetlands treated the domestic wastewater well, meeting the irrigation water quality standards for most water quality parameters with exception of phosphorus, ammonia-nitrogen, potassium and total coliforms, which showed high values significantly (p < 0.05) exceeding the thresholds set for irrigation purposes. The growth of both Sweet Pepper and Chilli fed with different treated and untreated wastewater types was assessed. A few plants suffered from either a shortage and/or excess of some nutrients and trace minerals. The overall growth development of Sweet Peppers was poor due to the high concentrations of nutrients and trace minerals. However, a high Sweet Peppers yield in terms of economic return (marketable yield expressed in monetary value) was linked to raw wastewater and an organic growth medium, while the plants grown in organic medium and irrigated with outflow from wetlands of large aggregate size, high contact and resting times, diesel-spill contamination and low inflow loading rate produced the best fruits in terms of their dimensions and fresh weights, indicating the role of diesel in reducing too-high nitrogen concentrations. In contrast, Chillies did reasonably well but the growth of foliage was excessive and the harvest was delayed. High Chilli yields in terms of economic return were associated with tap water and an organic growth medium, and a wetland with a small aggregate size and short contact time and long resting time with a low inflow loading rate, while the best fruit quality in terms of length, width and weight was observed for plants grown in organic media and irrigated with outflow water from wetlands containing small aggregates with long contact and resting times and fed with a high inflow loading rate (undiluted wastewater), releasing more nutrients into their effluent resulting in a greater marketable profit. Low fruit numbers correlated well with inorganic growth media. Filters contaminated with hydrocarbon were usually associated with a substantially lower Chilli marketable yield than those filters lacking hydrocarbon pollution. Chilli generations were grown successfully when using wastewater treated by constructed wetlands and organic soil. High Chilli generation yields in terms of economic return were associated with wetlands containing small aggregates with long contact and resting times and fed with a high inflow loading rate (undiluted wastewater), releasing more nutrients into their effluent producing the best fruit quality in terms of length, width and weight resulting in a greater marketable profit. Chilli generation plants were grown with considerably shorter heights and produced abundant fruit numbers which were harvested earlier than their mothers due to the reduction of irrigation water volume applied on them compared to their mothers. However, excessive nutrients applied on mother plants via irrigation water resulted in better fruit quality in terms of dimensions and weights compared with their generations, leading to a greater marketable profit. Findings indicate that nutrient concentrations supplied to the crops by a combination of compost and treated wastewater are usually too high to produce a good harvest. However, as the compost was depleted of nutrients after about ten months, the harvest increased for pots that received pre-treated wastewater. The productivity of crops in terms of harvest was independent of the wastewater consumption volume, but may have depended on the water quality. A high yield was related to the most suitable provision of nutrients and trace elements. The mineral content of the organic soil was significantly higher than that for the inorganic soil, before and after irrigation with treated wastewater. No substantial mineral contamination was observed in the soils due to irrigation with treated wastewater. Slight to moderate zinc contamination was detected in harvested fruits based on common standards for vegetables. No bacterial contamination was detected for fruits harvested from plants irrigated with wetland outflow water. In contrast, fruits harvested from those plants irrigated with preliminary treated wastewater showed high contamination by total coliforms, Streptococcus spp. and Salmonella spp. , especially for fruits which were located close to the contaminated soil surface. However, findings indicate that vegetables receiving wastewater treated with wetlands can be considered as safe compared to those receiving only preliminarily treated wastewater. The project contributes to ecological sanitation understanding by closing the loop in the food and water chain. Findings will lead to a better understanding of the effects of different wetland treatment processes on the recycling potential of their outflow waters.