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Recovery of Phosphorus from Sewage Treatment Works - Phosphorus

VIEWS: 23 PAGES: 4

									Phosphorus Recovery from Sewage Treatment Works
Executive Summary of MSc Thesis by Mathis Rogner
Centre for Environmental Policy
Imperial College London
Academic Year: 2008 – 2009
Supervisors: Nick Voulvoulis, Guy Ohandja

Objectives:
Five main objectives were identified:

    •   To identify the drivers for phosphorus recovery.
    •   To identify technologies for recovery from wastewater and sludge for further investigation.
    •   To compare recovered phosphorus products.
    •   To compare phosphorus recovery technologies given holistic set of comparison criteria.
    •   To evaluate and confirm best performing technologies by case study a case study.

Introduction
Phosphorus is one of the most essential elements limiting plant growth and therefore, crop yields.
Human activities have disrupted the natural cycle of phosphorus to a point where phosphorus rock
resources are diminishing, while at the same time, phosphorus is accumulating in the natural
environment, often with negative consequences.
Thus, there is a growing consensus among sanitary engineers that wastewater is not necessarily a
“waste”; rather a potential supply of valuable resources, one of which is phosphorus. There exist a
number of emerging technologies to recover phosphorus from sewage treatment works.

The fundamental aim of the project is to identify, evaluate and compare various technologies for
phosphorus recovery from wastewater or wastewater sludge. It aims to deliver a clear overview of the
strengths and weaknesses of the technologies investigated, to assist policy and decision makers in
deciding which recovery process to concentrate investment in further research and development.

Methodology
The first phase of this work involved the identification of drivers for phosphorus recovery by a review
of relevant literature. By reviewing scientific literature and existing and emerging E.U. and U.K.
legislation, the drivers for recovery were identified. A review of existing literature was also used to
identify and select the recovery technologies for subsequent comparison. The technologies identified
for comparison were pre-screened, in that their development needed to have progressed beyond the
lab-scale, and at least a pilot scale plant was required to have been constructed. The literature review
was the main method to collect information on the various possible recovered phosphorus products
and the performance characteristics of the technologies themselves. These included reports released
by the technology manufacturers themselves, as well as independent reviews of the technologies.

Both the recovered phosphorus products and the recovery technologies were compared using a
method based on Multi-Criteria Analysis. A holistic set of comparison criteria were determined, and
the products and technologies were compared using qualitative and quantitative rankings. The
products were compared based on their prospective use in agriculture, in the efficiency and ease of
recovery, and in their associated environmental benefits. Recovery technologies were compared
according to product quality (purity), cost, operational efficiency, and associated environmental
benefits. For each product and technology, the comparisons were optimised for each of the above
parameters, to show the how each option performs relative to the single parameter. It is important to
note that the wastewater and sludge technologies were compared separately as the nature of the
processes involved make a direct comparison impractical.

In order to evaluate and confirm the best performing technologies after comparison, a case study
using real data from the Whitlingham Trowse STW was performed. This involved using input flow
data and nutrient concentrations to model the amount of chemicals required to be added and the final
output of recovered products.

Results
Three main drivers were identified for phosphorus recovery. These were: sustainability and
environmental concerns, existing and emerging legislation, and economic drivers. In addition to the
well documented risk to the environment due to eutrophication, it was determined that the current
practice of extraction and use of phosphorus fertilisers is not sustainable. The current methods
produce hazardous and toxic by-products. Diminishing high quality reserves mean that the processing
will require more energy and will produce more by-products. Meanwhile, the agricultural demand for
phosphorus fertilisers is only expected to increase in the future.

Legislation regulating the discharge of phosphorus into surface and ground waters, and also regulating
the application of wastewater sludge to agriculture are typically in place to protect the environment
for discharges associated from STWs. It was determined that phosphorus discharge consents will
only become more stringent in the future, requiring more phosphorus to be removed to sludge.
However, stricter regulation on the applicability of sludge to land will also lessen the route for
agricultural sludge disposal. This provides an impetus to recover phosphorus separately thus
removing it from both wastewater and sludge.

Recovery technologies will only be viable if they can compete with the conventional processing of
phosphate rock. As discussed earlier, reserves are diminishing, thus increasing the price of phosphate
rock and also fertilisers. This allows for recovered phosphorus products to be competitive in the near
future.

The potential recovered phosphorus products were identified to be ferric phosphate, calcium
phosphate and struvite. In comparison, it was determined that struvite is the best product for use in
the U.K. As ferric phosphate cannot be utilised as a fertiliser or a raw feed in phosphate processing,
and while calcium phosphate can be a substitute raw material in phosphate rock processing, no
industry of this type currently exists in the U.K.

Of the wastewater technologies studied, it was determined that fluidised bed reactors and the
ViroFilter technology were the best performing processes over all parameters. Sludge technologies
compared resulted in KREPRO and Aqua-Reci, a sludge treatment technology based on supercritical
water oxidation, showed the most promise. A summary of these comparisons and their scores are
shown in Figures 1 and 2.

The case study modelled the use of fluidised bed reactors, ViroFilter, KREPRO and Aqua-Reci
technologies at Whitlingham Trowse STW. It was able to highlight the exorbitant chemical
requirement for sludge technologies. It did, however, show that the use of fluidised bed reactors
requires reasonable chemical addition, and can produce up to 232 g struvite per cubic meter
wastewater treated.
                 Optimisation Parameter       FBR     Unitika   REM-NUT      ViroFilter
                 No Weightings                 14       18           18          13
                 Product Quality               18       26           22          29
                 Cost                          23       32           37          17
                 Operational Efficiency/Use    23       23           33          15
                 Environmental Benefits        28       36           26          22
          Figure 1: Summary of Wastewater Technologies Comparison, Optimisation of Each Parameter


                 Optimisation Parameter       KREPRO      KEMICOND        AQUA-RECI
                 None                            14             11             13
                 Cost                            28             14             23
                 Product Quality                 20             27             21
                 Operation Efficiency/Ease       22             11             26
                 Environmental Benefits       21           19             23
               Figure 2: Summary of Sludge Technologies Comparison, Optimisation of Each Parameter



Discussion, Conclusions and Policy Implications

The decision to implement phosphorus recovery in a sewage treatment works will, inevitably, fall
upon the shoulders of the economic viability of the recovery process. However, increasingly stringent
regulations on phosphorus concentrations in both wastewater and sludge can provide further impetus
to develop phosphorus recovery technologies today.

The comparison of wastewater and sludge technologies yielded that when evaluated in within the
context of the U.K., wastewater technologies would prove to be a better investment. Sludge treatment
technologies are more expensive and require a lot of added hazardous chemicals. These technologies
were developed in countries where the disposal of sludge to land is severely limited or prohibited.
The U.K. still allows application of treated sewage sludge to agriculture, and as such, the main
limitations preventing application is the concentration of heavy metals and nutrient concentrations.
Thus recovering phosphorus from the wastewater stream, either directly after secondary treatment, or
after anaerobic digestion, allows for more sludge to be continually disposed of to land.

Of the wastewater treatment technologies, the fluidised bed reactors show the most promise. They are
well established in both Japan and in Canada and are proven to be able to produce a marketable
struvite product. They are also easily retrofitted to existing assets in STWs and can treat a variety of
different influents and flows. The ViroFilter technology is also very interesting in that it is a
completely passive operation, in that it does not require operator attention, and is ideal for smaller
unmanned STWs. However, further research is required to test the effectiveness of the expended
filter material as a fertiliser.

A major limitation of this study was the lack of reliable data. As many reports were issued by the
manufacturer or on their request, the information released was selective in that only positive aspects
of technologies were printed. It was very difficult to find reliable cost data as the technologies were
developed at different times in different countries. Manufacturers are also reluctant to release detailed
cost data to non-buyers. As the comparisons are relative this is not a big issue, however it limits the
value of the case-study as many gross assumptions were made.

Looking forward, the aim of this report is to give an overview of recovery technologies in a single
concise document. A technology should not be evaluated in isolation; rather, it should be compared
to the alternative options available. However, before a decision to construct new technologies in
STWs can be made, further research into the individual selected technology should occur. This would
be best performed by a life-cycle analysis. As the viability of a recovery technology relies on the
marketability of the recovered product, a market analysis is recommended to be undertaken to
evaluate whether or not the recovered product can be sold for profit within the U.K., or more
importantly, in the near vicinity of the STW itself.

								
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