INVERSIONES INMOBILIARIAS 14.1 Propiedades de inversión

This literature reviews current knowledge related to each research objective as detailed in section 1.3. The literature review begins with a brief overview of current technologies used to recover P from waste arising as a result of wastewater treatment. This acts to inform the reader of the main technologies used, and their benefits and disadvantages. Section 2.3 describes the current knowledge and associated research gaps relating to P recovery from sludge dewatering liquors, i.e. similar to the Ostara P recovery system. Section 2.4 discusses literature on the topic of mineralogy of sewage sludge before and after thermal treatment and following acid leaching.

2.2 Method of P recovery from WWTPs

There are numerous methods available for recovering P from the waste products of wastewater treatment: wastewater, sewage sludge and incinerated sewage sludge ash (ISSA). There are three common techniques for P recovery subsequently described; crystallisation & precipitation, wet chemical treatment, and thermo-chemical treatment.

2.2.1 Crystallisation & Precipitation

Crystallisation followed by precipitation is a common method of P recovery used by the Ostara, Airprex, and Crystalactor processes, outlined in Table 2-1. Crystallisation and precipitation transforms soluble phosphate from a liquid phase into a solid phase (Nieminen, 2010). Precipitation of P can occur

spontaneously, although it is normally initiated by the addition of metal ions, e.g. Mg2+_{, Ca}2+_{, Al}3+ _{or Fe}3+

(Rittmann et al., 2011a). Particles growth occurs either by agglomeration or by using seed materials such as sand (Pratt et al., 2012). Mg and Ca are frequently used to precipitate P as a fertiliser, as they are

2.6 References 2.5 EBPR Performance 2.4 P recovery from ISSA & PSSC 2.3 P recovery from sludge dewatering liquors

2.2 Methods of P recovery from WWTPs 2.1 Introduction

secondary nutrients required for healthy plant growth. When P is precipitated using Mg, struvite

(MgNH4Po4.6(H2O)) is formed at pH >8 (Rittmann et al., 2011a). This is the more common method of P

precipitation. Effective struvite precipitation involves crystal formation followed by separation from liquid (Pratt et al., 2012). P may also be precipitated using Ca in the form of hydroxyapatite (HAP)

(Ca5(PO4)3OH) (Rittmann et al., 2011a). However, this form of P precipitates at prohibitively high pH

values typically ≥ 10 (Rittmann et al., 2011a). Al and Fe are frequently used in WWTP to precipitate P from wastewater into an Al or Fe bound phosphate. However, P which is too tightly metal-bound is generally thought as unavailable for struvite precipitation and plant growth (Rittmann et al., 2011a).

Regardless, the KREPRO process precipitates P as ferric phosphate (see Table 2-1) (Rittmann et al.,

2011a).

2.2.2 Wet Chemical Treatment

Wet chemical processes release P from chemical or biological sludge and sludge ash using acids or bases (Nieminen, 2010). Generally, leaching with acids removes more P than base leaching (Nieminen, 2010). The LEACHPHOS and Seaborne processes each utilise wet chemical methods to recover P. Non-soluble residue is removed and the remaining liquid treated to separate dissolved P using method such as precipitation, ion exchange, and nanofiltration (Nieminen, 2010). The ion exchange process allows for undesirable ions to be exchanged for solid-phase ions based on ion affinity (Rittmann et al., 2011a). Ion exchange offers a more selective method of removing ions from wastewater or sludge (Rittmann et al., 2011a). This is a more promising technology as it is generally a reversible process (Rittmann et al., 2011a). All heavy metals (except Hg which evaporates at thermal sludge treatment) input into the process transfer into the final product (Egle et al., 2014).

2.2.3 Thermo-Chemical Treatment

Thermo-chemical treatments are suited to the removal of P and heavy metals from ISSA and sludge (Nieminen, 2010). Mono-incineration of sludge completely destructs the organic pollutants present in the waste (Adam et al., 2009). These incinerated residues contain high concentrations of P, but this P is not present in a bioavailable form for plant growth (Adam et al., 2009). In thermo-chemical treatment, P in the ISSA is transferred into a mineral phase more available for plant growth (Adam et al., 2009). Volatile heavy metal chlorides are formed by adding a Cl donor at temperatures between 800-1000°C (Adam et al., 2009). The volatile heavy metal chlorides are separated from the gaseous phase (Adam et al., 2009). New mineral phases are produced in the ash, resulting in P solubility of up to 100% in citric acid (Adam et al., 2009). However, due to the concomitant removal of metals with P, the use of the treatment in producing fertiliser is limited (Nieminen, 2010).

While there are many methods available to recover P from wastewater, sludge, and ISSA, many of these options are unattractive in terms of sustainability. Most of the processes produce waste which must be landfilled, while other processes require high energy and acid addition making them economically

unviable. Crystallisation processes are preferred as they achieve high P removal and recover P as useful product, i.e. struvite or calcium phosphate (Bhuiyan et al., 2008). However, struvite is favoured over calcium phosphate as the nutrients are released at a slower rate (Bhuiyan et al., 2008). The essential nutrients for plant growth P, N and Mg are precipitated simultaneously into the struvite fertiliser (Bhuiyan et al., 2008).

Each of the method described above are applicable to different wastes produced at WWTPs, i.e. wastewater, sludge, and ISSA. Table 2-1 provides a review of the current technologies used to recover P from each of these waste streams.

Table 2-1: Review of current technologies available for recovering P from WWTPs

Technology Process Final product % P recovery

Limitations Benefits References P recovery from wastewater/sludge dewatering liquors

Rim-Nut (Rem-Nut) Ion exchange, precipitation Struvite Up to 95% NaCl is needed to regenerate resins. P and Mg salts may be required to precipitate struvite. Complex process requires high operation skills Incidental removal of SS, BOD, COD Karapinar, (2009); Woods et al., (2013); Nieminen, (2010); Environment Agency, (2012) Ostara Crystallisation Struvite 80-90% PO4 > 75 mg/l is

required to achieve economic recovery of struvite

High purity slow release fertiliser product

Ostara, (2010); Environment Agency, (2012) Crystalactor Crystallisation Calcium

phosphate

70-75% Requires readily available lime to raise pH. Frequent injection of seed material is required as grains become part of final product High purity product. Can recover heavy metals Woods et al., (2013); Nieminen, (2010); Environment Agency, (2012) Struvia (Originally Phostrip) Crystallisation Struvite, calcium phosphate 11% of sludge input

Only applicable to EBPR WWTPs with PO4-P greater than 50mg/l in sludge liquor High purity product P-REX, (2015f)

P recovery from sewage sludge

Krepro Thermal hydrolysis, acid leaching, precipitation Ferric phosphate

~75% Process produces iron phosphate which is thought to be unavailable for plant growth

Suitable for both digested and raw sludge. Various products recovered: biofuel, phosphate, precipitant and carbon source Kemira Kemwater, (2001); Nieminen, (2010); Environment Agency, (2012)