Recent legislation in North America suggests stricter control will soon be put on the release of nitrogen and P-laden greenhouse wastewater into the environment (OGVG, 2012). Unlike nitrogen, conventional treatment systems for greenhouse wastewater, namely constructed wetlands, cannot effectively remove P (Vymazal, 2007). Concentrations of orthophosphate-derived P (PO4-P), the most common form of P in used greenhouse nutrient solution (which will be referred to as greenhouse wastewater), are typically around 30–60 mg·L−1 (Gagnon et al., 2010; MOE, 2012; Prystay and Lo, 2001), but can be as high as 370 mg·L−1 P (Saxena and Bassi, 2012). In Ontario, a total P limit of 1.0 mg·L−1 is usually imposed on industries discharging to surface water to prevent damage to aquatic ecosystems (MOEE, 1994), suggesting technology will be needed to drastically reduce [P] in greenhouse wastewater.
Phosphate precipitation using lime addition is one of the most basic and widely used methods for P treatment in other wastewater types (de-Bashan and Bashan, 2004; Pratt et al., 2012), and can achieve >99% soluble P removal in greenhouse wastewater (Dunets and Zheng, 2014). Interactions of fine precipitate with organic material cause issues with separation of precipitated P in other wastewater types (de-Bashan and Bashan, 2004; Pratt et al., 2012; Valsami-Jones, 2001). However, due to the typically low organic (Gagnon et al., 2010; Prystay and Lo, 2001; Saxena and Bassi, 2012) and very high nutrient content of greenhouse wastewater, there is potential to not only efficiently remove but also recover large amounts of P as largely pure calcium phosphate (Ca-P) precipitate that has great potential to be reused as fertilizer. Recovery is currently a major research interest as P is a valuable nonrenewable resource (Valsami-Jones, 2001). As greenhouse wastewater contains high concentrations of all plant nutrients, recovered precipitate could be a source of other nutrients (such as K, Mg, and micronutrients) in addition to Ca and P through substitution in the Ca-P mineral structure and formation of other precipitates (Saxena and Bassi, 2012; Yi et al., 2005).
However, potential issues with fine Ca-P precipitate separation and optimizing recovery while simultaneously meeting low P limits in a low-organic, high-P wastewater such as greenhouse wastewater have yet to be investigated. Depending on particle size and separation rate of fine precipitate, which may be affected by wastewater composition (such as carbonate and Mg content; Valsami-Jones 2001), accommodating extended settling time required for full separation may not be efficient. A combined method of precipitation followed by a flocculation step to gather fine precipitate into larger flocs and speedup settling could aid in streamlining the separation and recovery process. Readily available, low-cost biopolymers such as starch (Vandamme et al., 2009), chitosan (Roussy et al., 2004), various types of gum (Gupta and Ako, 2005), and alginate (Renault et al., 2009) are promising flocculant options as they are readily biodegradable and pose no environmental hazard in discharged wastewater.
A combined precipitation/flocculation separation process using lime and a low-cost, biodegradable flocculant must be investigated as a potential optimized P recovery method specifically suited to greenhouse wastewater. An optimal separation process should reduce total P in wastewater to low levels (<1 mg·L−1) required to prevent environmental damage, minimize P separation time, and retrieve a product with a nutrient composition that makes it valuable as a fertilizer. The aim of this study was to evaluate the capacity of different lime/flocculant combinations to meet these criteria by examining separation dynamics in real and model wastewaters of varying composition, as well as analyzing nutrient composition of the retrieved precipitate to determine viability for reuse.
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