Innovative production treatment hydroponic farm for primary municipal sewage utilisation
Introduction
By early next century there is likely to be an enormous demand for water reuse across various urbanised regions of the world to satisfy environmental, economic and social pressures. In well developed economies, this demand is most likely to arise from the need to improve environmental flows, thus allowing less water for human consumption and severely limiting the likelihood of new diversion dams. Additional pressure for specific communities to reuse wastewaters is also resulting from the shift to economic and financial management of their limited water resources.
A clear example of the environmental and economic paradigm in water management is the possible reduction in energy consumption (=greenhouse gases) by reducing the pumping of water and wastewaters across cities by the application of localised reclamation technologies. The application of hydroponics, however, has largely been neglected as an option, not only to treat wastewaters (Ayaz and Sagin, 1996), but also to produce value added crops.
Virtually every terrestrial plant appears to be capable of growing in some form of hydroponic system (Jewell, 1990, Cooper, 1996) where their roots are supported by a growing medium such as gravel or sand. Growing plants in a shallow flow of nutrient solution is known as nutrient film technique (NFT). Varying designs of the hydroponic system provides alternative support systems to differing NFT plants, as described by Cooper (1996).
Whereas wastewater irrigation is common practice in Israel, the Arab states, Arizona, California and Florida (Asano, 1987, Shuval, 1987, Arar, 1991), the potential advantages of hydroponic treatment for BOD, suspended solids (SS), nutrient and pathogen reduction appears to have been largely unstudied (Jewell, 1994). Soil based crops irrigated with wastewaters appear not to cause short-term (acute) ill effects (Pettygrove and Asano, 1985), although ground waters may accumulate harmful contaminants. In contrast, hydroponic applications should neither effect crops nor receiving waters so long as the biomass is appropriately utilised.
This paper therefore, investigated the possible human health impact from a worse case scenario, that is the growth and possible human consumption of lettuces grown in NFT of primarily municipal effluent. Further, to aid in the design of a conceptual layout for a production treatment hydroponic farm (PTHF), phosphorus was considered the limiting nutrient for plant growth and discharge quality for acceptable non-potable reuse applications.
Section snippets
Materials and methods
A commercial hydroponic system (Fig. 1) was adapted for the experimental trials. The system consisted of five PVC plastic channels 3 m long by 100 mm wide. Primary treated effluent was pumped from a 200-l tank to the head of the NFT channels for gravity feed via the lettuce plants in a closed-loop NFT configuration (wastewater plot). A commercial nutrients solution (Accent Hydroponics, Sydney) dissolved in tap water was pumped from a 60-l tank for gravity feed to the plants in an identical NFT
Lettuce growth rates
Growth rates from the conducted experimental trials are summarised in Fig. 2. The plants grown with the control nutrient solution generally showed the highest growth rates, possibly because of the relatively high concentrations of nutrients (e.g. K) used for the control plants (Table 1). Of the effluent mixes, the plants grown in the 1:1 effluent:water mix exhibited the highest growth rates, followed by the plants in the undiluted effluent, then plants in the 1:3 effluent:water mix. These
Conclusions
Though lettuce growth was inhibited by toxins and despite the low concentration of wastewater DO and potassium, crops were raised successfully and municipal effluent was effectively treated in the experimental NFT hydroponic system. Additional nutrients (e.g. potassium) may have to be added to primary treated effluent (depending on the availability in the utilised effluent) for better commercial crop yield.
This paper proposes a conceptual layout for a full-scale PTHF, taking into account the
Acknowledgements
The authors wish to thank Brace Boyden for instigating the work and the NSW Environmental Trust for financial support throughout the project. Furthermore, microbiological analyses were reliant upon the excellent work of Anna Carew and improvement in the P-modelling by Zdenko Rengel's are gratefully acknowledged.
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