by Nannette Woods/CH2M HILL
Liliana Maldonado/CH2M HILLGlen T. Daigger/CH2M HILL
An increasing demand for sustainable resources and environmental stewardship has prompted industry to explore alternatives to conventional phosphate removal technologies, including those aimed at phosphorus control. This trend has been observed in countries such as The Netherlands, Japan, Italy, and South Africa. As a result, research and development are ongoing in these and other countries to explore the feasibility, economics, and performance of phosphorus treatment technologies that provide for the recovery of phosphorus as phosphate in usable forms.
In North America, water quality based regulatory initiatives have been implemented to reduce phosphorus discharge to receiving waters. Two primary regulatory approaches have been used in both the municipal and industrial sectors to meet this goal: restrictions on the use of phosphorus based products (source control) and strict control of phosphorus discharges (effluent permit limits). Recovery methods have not been used in North America to any significant extent. Long-term trends indicate the potential for increased interest and use of beneficial reuse and recycle options in lieu of the traditional treatment and discharge approaches used thus far. However, phosphate recovery will have to compete economically with established methods of phosphate removal. This paper presents the economic boundary conditions where recovery becomes more attractive than the traditional treatment removal methods.
In February 1997, the Chemical Manufacturers Association Phosphate CHEMSTAR Panel commissioned CH2M HILL to assess the viability of implementing phosphorus recovery technologies at municipal wastewater treatment facilities in North America. The study focused on evaluating the benefits and costs of potential integration of phosphorus recovery technologies with existing phosphorus removal systems. In April 1998, the Panel expanded the study to include an evaluation of the economic boundary conditions to determine at what point recovery systems become viable. The specific objectives of these studies were to (CH2M HILL, 1997):
The focus of this technical paper is on the economic boundary condition evaluation. However, a brief overview of the other objectives is required for development of the model used for the economic boundary condition evaluation.
Status of Phosphorus Removal
In response to phosphorus reduction initiatives implemented in the U.S., two regulatory approaches have been successfully used to minimize phosphorus releases to the environment:
As shown in Figure 1, phosphorus reduction programs have been implemented in the U.S. on a regional basis, primarily in response to local water quality issues. In general, where such programs have been implemented, bans have been placed on the use of certain phosphorus-based products, such as detergents. Review of data from existing municipal facilities indicated that influent phosphorus concentrations in municipal wastewaters are in the range of 2 to 9 milligrams per liter (mg/L), with a median concentration of 6 mg/L noted.
Figure 1 Regional Implementation of Phosphorus Reduction Programs
Current methods typically applied at municipal wastewater treatment facilities to remove phosphorus include:
Process flow diagrams for representative wastewater treatment scenarios that apply these methods are presented in Figure 2.
Representative Municipal Wastewater Treatment Scenarios
The feasibility of implementing phosphorus recovery to compliment conventional methods currently employed at municipal wastewater treatment facilities was assessed using the following drivers:
Recovered and reused phosphorus products could potentially be used in lieu of phosphate ore as a raw material for many phosphorus-based products, including fertilizers. This would, in turn, provide a venue for improved stewardship of phosphorus-based products.
Selection of Representative Recovery Technologies
Review of the available literature resulted in the identification of 10 primary phosphorus recovery technologies. Six of these processes were selected for detailed literature review because they were considered to be representative of the major technologies identified in the literature. The detailed literature review addressed various criteria, including categorization of each process in terms of process chemistry, process application, and demonstration level. Using the demonstration level as a key indicator of the success of previous research and development efforts, the Crystalactor, Phosnix, and Rim-Nut processes were selected for detailed economic evaluation. These technologies were viewed as having the greatest potential to be implemented. Process flow diagrams for these technologies are presented in Figures 3 through 5.
Crystalactor Process Flow Diagram
Phosnix Process Flow Diagram
Rim-Nut Ion Exchange Process Flow Diagram
The Crystalactor process was selected for detailed evaluation as a representative method of calcium phosphate crystallization for both tertiary and sidestream treatment applications. As a tertiary treatment method, this process could potentially replace currently employed tertiary lime addition methods, offering the benefits of reduced lime usage and sludge generation. As a sidestream treatment method, the potential benefits of improving mainstream treatment performance and sludge quality were evaluated. For each application, the recovered calcium phosphate yield and value were assessed.
The Rim-Nut ion exchange process was selected for detailed evaluation as a representative method for recovering magnesium ammonium phosphate (MAP) via tertiary treatment. Full-scale implementation of this technology has been demonstrated, but the application focused on ammonia removal. For this study, the focus was on the phosphate recovery aspect. As a tertiary treatment method, its effluent characteristics, robustness, and reliability were assessed along with the recovered MAP yield and value.
The Phosnix process was selected for detailed evaluation as a representative method for recovering MAP in a sidestream treatment application. Full-scale implementation of the process has been demonstrated. As a sidestream treatment method, the potential benefits of improving mainstream treatment performance and sludge quality were evaluated, as well as the recovered MAP yield and value.
Basis of Evaluation
Representative phosphorus recovery methods were evaluated using the following approach:
Representative Municipal Wastewater Treatment Scenarios with Phosphorus Recovery
The best case approaches for implementing phosphorus recovery technologies at municipal wastewater treatment plants were evaluated as represented by the following scenarios:
The economic analyses were conducted using the following variables:
|Chemical Cost Variables|
|Cost ($/Metric Ton)|
|50% Sodium Hydroxide||220||275||330|
The range of influent phosphorus concentrations used were higher than is currently observed in the United States. An influent phosphorus concentration of 6 mg/L was the mean concentration found in a recent survey of medium to large municipal wastewater treatment facilities in the United States (AMSA, 1995). The variables of 10 and 20 mg/L influent phosphorus were selected to provide a range of concentrations. A 10 mg/L phosphorus concentration is representative of a wastestream in a North American municipality, which has minimized inflow and infiltration. The higher value of 20 mg/L provides a "high-end" range for the model and bounds the concentrations that may be encountered in other countries.
The range of sludge handling costs covers the range of costs obtained in a survey of municipal wastewater treatment facilities in the United States (AMSA, 1995). The median cost from this survey is $54 per metric ton.
The evaluations were conducted using an effluent phosphorus concentration of 1 mg/L and a flow rate of 50 mgd. An effluent phosphorus concentration of 1 mg/L is a typical permit standard in areas of the United States, where phosphorus limits are set. The flow rate of 189,250 m3/day is the median flow rate at medium to large municipal wastewater treatment facilities in the United States (AMSA, 1995).
The spreadsheet modeling process begins by completing a mass balance for each technology scenario. The mass balance calculations were developed primarily to quantify, on a relative basis, the factors that could influence the economic viability of phosphorus recovery technologies and are not intended to represent a rigorous analysis of the biochemistry of phosphorus removal.
The mass balance information was used to determine chemical requirements, sludge volume generated, and recovered phosphate volume generated. The costs associated with these materials and products were then determined. The sum of the chemical costs, sludge handling costs were subtracted from the recovered phosphate value to obtain the major operating costs associated with phosphorus recovery or removal. The major operating cost was subtracted from the major operating cost for conventional phosphorus removal (either biological or chemical) to obtain the net operating savings. The results of these calculations are provided in Tables A-1 through A-6 in the Attachment.
To put the cost savings discussed below in perspective, a typical 189,250 m3/day plant might spend from $7 to $9 million per year on operation. Of this total operating costs, sludge operations might range from $0.3 to $5.0 million per year.
Relative to the influent phosphorus concentration and sludge handling cost, chemical cost has minimal impact on the economic viability of the phosphorus recovery technologies. Figures 7 and 8 provide examples of the influence of chemical costs on the net operations savings for a biological treatment system with a sidestream Crystalactor system and a Chemical treatment system with a mainstream Rim-Nut system, respectively. The net savings only changed slightly, compared to the impact of sludge handling costs for the Crystalactor system. Only those technologies requiring large volumes of chemicals, such as the Rim-Nut process, were greatly affected by changes in chemical cost. It should be noted that the value of the recovered phosphate is very small relative to the chemical costs in all scenarios evaluated.
Figures 7 and 8 also illustrate the dramatic influence the sludge handling costs can have on the net operations savings calculated. For example, at high sludge handling costs, the Rim-Nut process results in overall savings, whereas at low sludge handling costs, the process results in increased operating costs.
The influence of influent phosphorus concentration on net operations savings is illustrated in Figure 9. The impact on savings was most dramatic for the mainstream Crystalactor process. This is due to the high chemical cost for conventional chemical treatment at high influent phosphorus concentrations. The cost of the conventional chemicals is a direct function of the phosphorus concentration, whereas the chemicals for the Crystalactor process are independent of phosphorus concentration. The net cost actually increases (negative savings) with the phosphorus concentration for the Rim-Nut process. The chemicals required for this process are also a direct function of the phosphorus concentration. Although not shown, the same is true for the sidestream Phosnix process.
Comparison Between Technologies
A comparison of the major operating costs between the technologies is provided in Figure 10. The costs for conventional biological and chemical phosphorus removal are included for comparison. The costs associated with chemical additions for the Phosnix and Rim-Nut ion exchange processes are generally greater than the cost savings that can be realized from reductions in sludge generation and the sale of the recovered phosphate product. Therefore, the Crystalactor process should be considered the most economically attractive technology for phosphorus recovery when considering operating costs. The cost savings of the Crystalactor system is greatest when comparing the mainstream Crystalactor system to the conventional chemical system than for the side stream Crystalactor system compared to conventional biological treatment. There are no net operations savings in using the Crystalactor process when compared to the conventional biological system.
Capital cost estimates for implementing the Crystalactor process for phosphorus recovery are presented in Table 2. Cost estimates for the Crystalactor process equipment were provided by DHV Water BV (The Netherlands). The raw equipment costs appear in parentheses. Based on engineering experience with similar projects and professional judgment, costs for ancillary facilities, support equipment, and installation were estimated as a percentage of the equipment cost. These are order-of-magnitude costs expected to be accurate within a range to +50 to -30 percent.
|Capital Costs for the Crystalactor Process|
|Installed Cost ($ million)|
$15 to $20 ($10.9)*
$40 to $45 ($30.7)*
costs, noted in parentheses, were provided by DHV Water BV, The Netherlands.
Table A-7 presents the Crystalactor process scenarios that provide an estimated payback period of less than 20 years. The payback period was estimated as the number of years required to recover the cost of installing the process given the potential net operating savings reported. This information is also illustrated in Figures 11 and 12 for the two Crystalactor processes. These figures suggest that the application of phosphorus recovery technologies becomes viable (payback period of less than 5 or 10 years) only at high influent phosphorus concentrations and/or high sludge disposal costs.
Payback Period Chemical with Sidestream Crystalactor
These findings are consistent with anecdotal information provided by DHV Water BV which indicated that, in their experience, the Crystalactor process has not been found to be cost-effective at influent phosphorus concentrations below 10 mg/L.
Besides economics, other drivers could positively impact the potential applicability of the technology in the future. These drivers might include legislation or regulations affecting sludge disposal options and/or costs; demands for sustainable resources; reuse or recycling initiatives; and changes in market conditions.
Payback Period Chemical with Mainstream Crystalactor
The following conclusions were reached as a result of this study:
This study focused on cost-effectiveness as the key requirement for potential marketability of phosphorus recovery. Based on this requirement, phosphorus recovery does not appear to offer widespread applicability in North America at this time. Specific conditions may exist where cost-effectiveness may be possible. For example, if in a large urban area sludge disposal costs were to increase substantially (up to the $770 per metric ton range) and water conservation was implemented (increasing the influent phosophorus concentration to greater than 10 mg/L), the payback period could fall to the 5- to 10-year range and thus be potentially viable for a municipality. In addition, phosphorus recovery from agricultural wastes, which may have low flows and high phosphorus concentrations, may be economically viable in some situations.
Broad-based incentives may also be necessary to increase the attractiveness andor demand for implementing phosphorus recovery technologies. Such incentives may take the form of societal demands for sustainable resources and/or regulatory agency actions that impose recovery or reuse requirements on municipal and industrial wastewater treatment providers.
The authors would like to thank Shawn Sock and Tom Simpkin/CH2M HILL for assisting with this project and the Chemical Manufacturers CHEMSTAR Phosphate Forum of North America for their funding and technical assistance.
|Albright & Wilson, Glen Allen, Virginia, USA|
|Albright & Wilson, Americas, Mississauga, Ontario, Canada|
|FMC Corporation, Princeton, New Jersey, USA|
|Rhodia a Rhône-Poulenc Company, Cranberry, New Jersey, USA|
|Solutia, Inc., St. Louis, Missouri, USA|
For more information regarding the CHEMSTAR Phosphate Forum of North America contact Kathy Marshall at 703-741-5619 or via E-mail at email@example.com
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Phosphorus Recovery EvaluationñReport of Findings. Prepared for Phosphate CHEMSTAR Panel (CMA Reference Number PHOS-1.0-RRR-CH2M). CH2M HILL. September 1994.
The AMSA Financial Survey. 1995.
"Chemical Prices." Chemical Market Reporter. Week ending February 14, 1997.