Elsevier

Water Research

Volume 34, Issue 3, 15 February 2000, Pages 825-834
Water Research

Innovative production treatment hydroponic farm for primary municipal sewage utilisation

https://doi.org/10.1016/S0043-1354(99)00231-6Get rights and content

Abstract

The objectives of this work were to investigate a conceptual layout for an inexpensive and simple system that would treat primary municipal wastewater to discharge standards. Furthermore, the system may provide an increased supply of safe water for irrigation with low operational costs and produce commercially valuable plants for small communities in arid and semi-arid areas. A commercial hydroponic system was adapted for this study and consisted of five gullies, 3 m long by 100 mm wide. Primary treated effluent was used to irrigate lettuce in one series and a commercial nutrient solution was used to irrigate the same type of lettuce in another series as a control, both by nutrient film technique (NFT). Nutrient and suspended solids were efficiently removed by the NFT plant system. While no uptake of F-RNA bacteriophages were detected within lettuce leaves, uptake was apparent from spiked virus-sized particles (fluorescent 0.1 μm microspheres) and equivocal from spores of the faecal bacterium, Clostridium perfringens. Microbial data was used in a β-Poisson dose response model and indicated that the probability of infection for a single ingestion event of NFT grown lettuce grown on primary treated municipal effluent was about 1.7% for viruses. Moreover, plants accumulated heavy metals in leaf tissues at concentrations higher than the maximum recommended levels for Australian and New Zealand food (As=6.5, Cd=3.8, Pb=20 mg kg−1). Hence, it is recommended to evaluate ornamental or non-edible crops, such as essential oils, pyrethrum or flowers for sewage treatment. A conceptual layout for a full-scale production treatment hydroponic farm (PTHF) for small communities was based on modelling phosphorus removal with the hydroponic NFT experimental pilot plant. With NFT culture of lettuces, roots and other surfaces accounted for 67–72% of total phosphorous (TP) removal (by adsorption mechanisms). Based on empirical modelling, an influent TP 2–6 mg l−1 PTHF would be expected to be economical for small communities (<400 people) and produce effluent with TP <0.15 mg l−1, SS <2.5 mg l−1 and BOD <55 mg l−1. Lower values would be expected if the effluent was polished through a humus filter.

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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