RESERVAS TECNICAS 25.1 Reservas para seguros generales

The earliest available literature reporting experiences of full scale P recovery at a WWTP was written by Ueno & Fuji (2001). The authors list the following as their reasons for the installation of P recovery system in their WWTP:

 Lower effluent P concentrations by reducing the P load returned to the head of the works;

 Reduce the amount of chemical coagulants used, which will in turn reduce the volume of sludge

produced;

 Produce a P rich material which could be used by the fertiliser industry.

These motivations are consistent with the reasons for the installation of the Ostara recovery system in Slough WWTP. However, these are expected effects of P recovery rather than realised effects.

Britton et al. (2009) listed the following as their expected benefits of their pilot-scale Ostara reactor treating 20% of the supernatant stream:

 20% reduction in PO4-P load;

 5% reduction in NH4+ load;

 Increased wastewater alkalinity due to caustic addition;

 Decreased alkalinity demand during nitrification due to reduced ammonia load;

 Reduction or potential elimination of struvite scale in pipelines.

Again these are expected outcomes and not quantified in the study WWTP. To enhance the uptake of P recovery in WWTPs the true effects of P recovery should be evaluated and reported. Without proof of operational benefits achieved by P recovery water companies are likely unwilling to install new technologies.

Vadiveloo et al. (2012) listed the benefits and drawbacks of removing P using the Ostara system compared to using ferric salts to control struvite formation (Table 2-2). Viewing the details in Table 2-2, P recovery using the Ostara system seems to have more advantages over ferric dosing to control struvite precipitation. Unreported in this table is the fact that treatment of the sidestream using the Ostara process reduced struvite precipitation by >40% (Vadiveloo et al., 2012). It was concluded that the Ostara P recovery system is a feasible and viable alternative to control struvite formation in Vadiveloo et al.’s test WWTPs. This research focussed on the potential direct effects of P recovery, the advantages/disadvantages of P recovery can be extended further to include indirect and directly realised costs, savings, effects on WWTP processes, etc.

Table 2-2: Advantages & disadvantages of long term struvite precipitation alternatives (Vadiveloo et al., 2012) Long Term Alternatives Advantages Disadvantages Ferric Addition

Lowest capital cost Highest 20 year present worth cost

Operator familiarity Increased P concentrations in biosolids which can reduce land application potential Can improve centrifuge solids capture H&S issues due to chemical handling Ostara Pearl Lowest 20 year net present worth cost Highest capital cost

Lowest operations and maintenance costs Larger footprint Lower P concentrations in biosolids which can

increase land application potential

New chemicals needed Sustainable solution, conserving finite natural

resources

Ferric cannot be used to improve solids capture rate

Bilyk et al. (2011) produced Table 2-3 detailing the capital, operating, and net present worth for scenarios in which Ostara, Multiform Harvest, and alum dosing are utilised at two different WWTPs, A and B. Although alum has the lowest initial capital costs, over a 20 year period it is not as cost effective as struvite precipitation (Bilyk et al., 2011). The elevated Ostara cost over Multiform Harvest includes the construction of a new building to house the Ostara reactor and its recirculation pumps, drying, and bagging equipment (Bilyk et al., 2011). The Multiform Harvest process produces unrefined struvite which does not require a drying and classifying system (Bilyk et al., 2011). Costs savings compared against alum dosing include; chemical cost savings, less sludge production, less methanol and oxygen demand in the EBPR process (Bilyk et al., 2011). While this is a comprehensive analysis, the data is theoretical rather than achieved in the WWTP. To encourage the uptake of P recovery it is important that the actualised benefits be reported also.

Table 2-3: Ostara, Multiform Harvest, and alum dosing CAPEX, OPEX, and present worth costs (Bilyk et al., 2011)

Ostara Multiform Harvest Alum Dosing

WWTP A B A B A B

Capital Costs $4,071,500 $4,371,500 $1,081,000 $1,381,000 - $690,000 Operating Costs -$14,281 -$35,958 $47,900 $47,900 $180,400 $324,800 Present Worth Cost $3,894,000 $3,924,000 $1,678,000 $1,978,000 $2,249,000 $4,738,000

Maaß et al., (2014) quantified that the installation of the AirPrex P recovery system generated an added- value of about €416,000 in their WWTP. The majority of this added-value came from reduction of operating costs and flocculating agents required in the WWTP, as shown in Figure 2-1. The revenue from sale of struvite fertiliser amounted to only 4% total benefits for this WWTP (Maaß et al., 2014). Many of the costs and benefits will be similar to those found in Slough WWTP, with the exception of MgCl salt,

which is provided by Ostara. A similarly low % revenue benefit will be calculated for Slough WWTP, because the majority of benefits come from onsite savings, rather than revenue. It would be interesting to compare and contrast the costs and savings as expected in Slough WWTP against the results reported by Maaß et al., (2014). This would create further evidence for the benefits of P recovery in WWTPs.

Figure 2-1: Costs and benefits of struvite production according to Maaß et al., (2014)

Similarly to Slough WWTP, Miami-Dade Water and Sewer’s Department (MDWASD) is adding ferric

sulphate (Fe2(SO4)3) to the centrifuge influent line as a short term measure to reduce nuisance struvite

precipitation in their WWTPs (Vadiveloo et al., 2012). The solution has been recognised as costly, with ferric sulphate costs amounting to an estimated $47,000 per month (Vadiveloo et al., 2012). It is unclear what parameters were used to determine this estimated cost per month when using ferric sulphate dosing, i.e. sludge handling.

One of the benefits described in literature, but unreported by Vadiveloo et al. (2012) and Maaß et al., (2014) is the improvement in the EBPR process in WWTP as a result of P recovery. The immediate benefits of P recovery are higher factor of safety on nitrification and EBPR, reducing stress on the process to allowing it to remove nutrients to a lower level than previously achieved (Bilyk et al., 2011).

The reduction in P and NH4 load is said to improve the P removal capacity and reduce the demand for

readily biodegradable chemical oxygen demand (rbCOD) for phosphate accumulating organisms (PAO),

allowing more rbCOD to become available for denitrification (Britton et al., 2009). The reduced NH4+

these effects has been reported (Britton et al., 2009). Baur (2010) report the reduced recycled P through the WWTP lowered P load to EBPR, making it more stable.

Along with reducing P recycled in the WWTP, P recovery reduces the concentrations of P in the sludge (Baur, 2010). Biosolids application rates to land are based on the amount of N/acre, with P typically being in excess of plant requirements (Baur, 2010). A reduced concentration of P in sludge, when spread on land, lessens excess P in soils thereby reducing eutrophication risk.

Britton et al. (2005) reported further reaching benefits of P recovery from WWTPs. Struvite production results in a 50% reduction of sulphur dioxide, carbon monoxide, and nitrous oxide and 80% less greenhouse gases production compared to traditional fertiliser manufacture (Britton et al., 2005). These reductions result from the fact that traditional fertiliser manufacture is energy intensive, involving mining, transport, thermal processes and direct combustion of fossil fuels (Britton et al., 2005).