Original Research Article

Wastewater Management and Re-use from a Nitrogen Based Fertilizer Complex

Dr. Hussameldin Ibrahim,
Jagafa Ishaya and Hussameldin Ibrahim*
Engineering and Applied Science, University of Regina, 3737 Wascana Parkway, Regina,
SK, S4S 0A2, Canada.

As a resource, water is often overlooked in Canada due to its plentiful availability and low cost. In the nitrogen fertilizer industry waster is used in many areas such as steam generation, product dilution and process stream cooling.

*Corresponding author:

Hussameldin Ibrahim,
hussameldin.ibrahim@uregina.ca

Keywords:

Wastewater management, ammonia, urea, modeling and simulation, SuperPro Designer, economic analysis

As a resource,water is often overlooked in Canada due to its plentiful availability and low cost. In the nitrogen fertilizer industry waster is used in many areas such as steam generation, product dilution and process stream cooling.Wastewater is generated a as by-product of these processes and often stored in retention ponds to evaporate over long periods of time. A treatment and management scheme was applied to such wastewater from a nitrogen based fertilizer manufacturer. The developed scheme was modeled and simulated using SuperPro Designer with the goal of obtaining water of a quality suitable for reuse in plant operations. The simulation indicated a recovery of 60% water and a total volume reduction of 96% with 4% waste (brine). The economic viability of the proposed treatment was also evaluated and compared with the costs of traditional evaporation ponds. The proposed process was found to be profitable with a net present value of $1,404,073 considerably higher than the conventional evaporation pond.

As a resource,water is often overlooked in Canada due to its plentiful availability and low cost. In the nitrogen fertilizer industry waster is used in many areas such as steam generation, product dilution and process stream cooling.Wastewater is generated a as by-product of these processes and often stored in retention ponds to evaporate over long periods of time. A treatment and management scheme was applied to such wastewater from a nitrogen based fertilizer manufacturer. The developed scheme was modeled and simulated using SuperPro Designer with the goal of obtaining water of a quality suitable for reuse in plant operations. The simulation indicated a recovery of 60% water and a total volume reduction of 96% with 4% waste (brine). The economic viability of the proposed treatment was also evaluated and compared with the costs of traditional evaporation ponds. The proposed process was found to be profitable with a net present value of $1,404,073 considerably higher than the conventional evaporation pond.

Fertilizers are typically classified according to the type of nutrient they provide such as nitrogen (N), phosphorus (P), or potassium (K). Such fertilizers are "straight fertilizers" and the nutrients they provide are macronutrients needed for plants’ growth. Multi-nutrient fertilizers or "complex fertilizers" provide two or more nutrients, such as NP, NK, and PK. Globallythe fertilizer industry has an estimated value of $171.6 billion,an increase of 31.9% since 2010and in 2014 shipped 183.5 million tons of fertilizer, an increase of 13.3% [1].According to recent statistics, by the year 2050 the projected world population is estimated to be around ten billion, an increase of three billion people from its current seven billion [2]. The Asian population, in particular, is expected to rise by 54% by the year 2050 and the African population is estimated to rise by 25% by 2050 [3]. With such significant rise in population the demand for food grains will also escalate. To meet the higher demand, fertilizer production has been on a steady rise over the past decades. The annual fertilizer use is estimated to be about 220 million tons by 2025, with a ±20% variance depending on the improvement in the efficiency of fertilizer quality and land application. Data from PotashCorp [2] has also showed that nitrogen based fertilizer, which currently accounts for more than 60% of the total fertilizer production and consumption will rise by about 20% by the year 2025. This greater need for fertilizer production inevitably creates more demand for resources such as energy (hydrocarbons, electricity), water, land and workers.

The Canadian fertilizer industry is a multi-billion dollar industry that contributed approximately $5.7 billion and $2.9 billion to the Canadian economy from the sales of potash and nitrogen fertilizers respectively, which accounts for 0.6% of Canada’s GDP [1]. The significant margin for profit available to the Canadian fertilizer industry is a result of the availability of cheap raw materials such as water and natural gas as evidenced from the locations of such fertilizer production plants in Western Canada. The greater consumption of resources such as water generates byproducts, wastewater and the issue of how to manage, treat and/or re-use it. The most widely adopted wastewater treatment method used in the nitrogen fertilizer industry is the use of dug out evaporation ponds due to their relatively low cost [4-7]. The evaporation pond is used mainly to further concentrate the contaminants present in the wastewater. Table 1 gives a summary of the common wastewater treatment methods and their key indicators (capital cost, operating cost, complexity and waste generation rate).However, evaporation ponds have some serious limitations. They have large area requirements, emit foul smells, have health risks to animals, contribute to environmental pollution, have a slow rate of evaporation (especially in cold climates) and they do not promote sustainable conservation of water.

The overall goal of this paper was to manage the wastewater generated from a typical nitrogen based fertilizer plant to recover a portion for re-use in plant processes. In this study, a simple wastewater treatment process configuration that significantly reduces the need for evaporation ponds was developed and modeled.  A process simulation model was developed and used to determine the feasibility of wastewater re-use in a typical nitrogen based fertilizer plant for the proposed configuration. The model uses a new proposed process configuration was implemented and optimized using the commercial software SuperPro Designer by Intelligen Inc.  This software provides unique capability of not only sizing the desired unit operations that make up the process flow diagram, but it also provides dynamic simulation capability of the process as well as the ability estimate pricing for the associated unit operation based on a library of industry benchmark. It also provides the ability for the user to input user-defined parameters as desired. Real world data of water consumption and wastewater generation was obtained from a prominent Canadian nitrogen fertilizer plant in Saskatchewan conditional of keeping the identity concealed.

Table 1: Comparison of wastewater treatment alternatives
Method Capital Cost O&M Cost Complexity Waste generation
Chemical dosing Low High Low High
Evaporation ponds Low Low Low Low
Hauling off site High Low Low Low
Deep well injection High High Low Low
Nano/Ultra Filtration High High High High

Figure 1 shows a typical schematic diagram of a nitrogen fertilizer production plant. Nitrogen fertilizer precursors include ammonia (NH3), sulphuric acid, phosphoric acid and nitric acid while final products include ammonium nitrate (AN), urea, ammonium sulphate, diesel exhaust fluid (DEF) and urea ammonium nitrate (UAN)[8]. Granular urea (NH2CONH2) is by far the most common form of nitrogen based fertilizer available on the market and is of great importance to the agriculture industry of Canada and the rest of the world. The process involves reacting NH3 and carbon dioxide (CO2) at high temperature and pressure to produce what is commonly known as urea melt. The urea melt is concentrated to 98%before granulation to produce urea pellets.

Clyto Access

Figure 1. Nitrogen fertilizer production routes [9]
Process design basis

The proposed design is based on evaporating 50-100 m3/hr of water taken from the evaporation pond. Water temperature is about 20 °C, pH 8.2 and at atmospheric pressure. The contaminants in the wastewater are given in Table 2.

Constituent

Value

Units

Conductivity

8.18

[mS/cm]

pH

8.2

[pH]

Carbonate Alkalinty

1

[mg/l]

Bicarbonate Alkalinity

525

[mg/l]

Total Alkalinity

430

[mg/l]

Total Dissolved Solids

6350

[mg/l]

Chloride

2110

[mg/l]

Sulphate

1770

[mg/l]

Calcium

89

[mg/l]

Magnesium

260

[mg/l]

Potassium

51

[mg/l]

Sodium

1460

[mg/l]

Ammonia

210

[mg/l]

Nitrate

310

[mg/l]

Nitrite

0

[mg/l]

Total Kjeldal Nitrogen, dissolved

220.0

[mg/l]

Total organic carbon

38

[mg/l]

Total coliform

46

[ct/100ml]

Fecal coliform

3

[ct/100ml]

Silicon

0

[mg/l]

Fluoride

1.1

[mg/l]

Arsenic

6.3

[µg/l]

Aluminum

0.086

[mg/l]

Cadmium

0.00002

[mg/l]

Chromium

0.0012

[mg/l]

Copper

0.0067

[mg/l]

Iron

0.037

[mg/l]

Lead

0.0001

[mg/l]

Manganese

0.0062

[mg/l]

Molybdenum

0.048

[mg/l]

Nickel

0.0049

[mg/l]

Selenium

0.0025

[mg/l]

Vanadium

0.081

[mg/l]

Zinc

0.0087

[mg/l]

Table 2: Characteristics of wastewater [10]
Process description

Figure 2 depicts the proposed process flow diagram. Wastewater from the pond is pumped through a series of heat exchangers to raise its temperature from 20 °C to 65 °C. Before the water is passed through the heat exchangers, it undergoes a pre-treatment stage. The first pretreatment step uses chemical coagulants and flocculation agents such poly-aluminum chloride (PAC) to help remove suspended particles, which can foul the heat exchangers and reduce their efficiency. The pre-treatment process starts with a clarifier, which helps flock settles out. The pH is adjusted to 6 by the addition of sulfuric acid (H2SO4) or hydrochloric acid (HCl) to keep the salts in solution in the evaporator as the salt concentration rises. The final pretreatment step is filtration using granular media such as gravel and sand layer. This helps further remove other particles, which did not settle out in the clarifier. Medium pressure steam (7 bar) taken from the plant at a temperature of 240 °C is used in the multi-effect evaporator to reduce the volume of water by up to 96%.The Vapor from the first stage is used in the next stage for a 3-stage multi-effect. 60% of the water is recovered as condensate, 34% as vapor and 6% as concentrated brine.

Clyto Access

Figure 2. Proposed wastewater treatment process flow diagram
Process simulation

The proposed wastewater treatment scheme was modeled and simulated using SuperPro Designer. Variable parameters used in the model were: flow rate, temperature, and heat exchange area. Fixed parameters included pH of water and initial temperature. Water flow rate ranged between 50 -100 m3. This choice of the range was based on the total volume of water stored in the pond (1,600,000 m3) and wastewater generation rate of 25 m3/hr in the plant. The flow rate had negligible effect on the overall plant cost, but does affect steam demand. The amount of coagulant and flocculent is set at a concentration of 10 mg/L. This ratio was found to be the most optimal based on the pH of 8.4. The clarifier is estimated to have a total surface of 75.4 m2 to handle 100 m3/h of water with a retention time of 135 min. Prior to introducing the water the granular media filter pH is adjusted from 8.4 to 7.6 by the addition of 5 mg of H2SO4 acid per m3 of water. Reducing the pH ensures optimal performance of the granular filter, heat exchanger and evaporator by preventing precipitation.

Source and characteristics of wastewater

In a nitrogen fertilizer plant, water is used for producing steam, product dilution, cooling, cleaning and firefighting. Typical sources of wastewater are: boiler blowdowns, cooling tower blowdowns, raw water treatment waste, storm water, exchanger backwash, process condensate and compressor blowdowns[11-13]. The quantity and quality of wastewater generated typically depends on the quality of fresh water, fresh water treatment process, age of the plant and the source stream of wastewater. The characteristics of each waste stream vary, with the boiler and cooling tower blowdown effluent having high concentration of salts[12,13]. Process condensate is typically laden with ammonia and/or urea while fresh water treatment (demineralization process effluent) has high salts, organics, heavy metals, carbonates, etc. Average production of wastewater is about 8-40 m3/tonne of urea produced. Table 2 shows typical values of contaminant concentrations found in a combined stream of wastewater stored in evaporation ponds.

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI) and the Clean Energy Technologies Research Institute (CETRi) of the University of Regina. Also, the authors thank the staff of Saskatchewan Minerals and Mosaic Potash for donating raw materials to be used in this study.

Considering a plant with a maximum processing capacity of 100 m3/h, the effect of flow rate, recovered heat and pH on the quality of recovered water were considered but found to be negligible and no direct correlation between the reported parameters and quality of water was found. The quality of recovered water is directly related to the feed water quality and plant processing capacity. Hence, the effects of these parameters on water quality were not considered further and discussions in this section were limited to the economic aspect of the study. The following presents the simulation results in terms of the effect of flow rate, temperature and pH on proposed process overall performance and economics.

Effect of flow rate

The wastewater flow rate directly impacts the size of unit operations such as: pump size, size of clarifier, heat exchange area requirement, steam demand and other consumables. Figure 3 shows the effect of flow rate on proposed process capital and operating costs. Values are expressed in millions of dollars. The results show direct correlation between costs and flow rate indicating that the higher the flow rate, the greater the cost. The figure also shows that the capital cost,exhibited higher sensitivity to changes in flow rate compared to the operating cost.It can be observed that the capital cost rises exponentially at higher flow rates with a ~$16 millions at 100 m3/h while operating cost maintained a steady and slow increase with ~$4/yr at 100 m3/h.

Clyto Access
Figure 3. Effect of flow rate on overall process cost

Figure 4 shows the effect of flow rate on the surface area requirements for the process heat exchangers. Values of surface area are expressed in terms of square meters. In general, the results show that all exchanger’s surface area is proportional to flow rate, which also corresponds to higher overall costs. The evaporator showed the highest sensitivity of surface area to changes in flow rates, followed by heat exchanger 1, clarifier and heat exchanger 2. The evaporator surface area increased by 240% as flow rate increased from 30 to 100 m3/h. Also, the evaporator surface area profile follows a similar trend to that of the capital cost given in Figure 3 suggesting that it is the largest contributor to total process cost.

Clyto Access

Figure 4. Effect of flow rate on the surface area of heat exchangers

Steam plays an important in the evaporation scheme by which the proposed wastewater treatment operates. Figure 5 shows the effect of flow rate on the steam required for evaporation. The results show that steam requirement is proportional to flow rate. Values of steam demand are expressed in terms of thousands of metric tonnes. The steam demand grew rapidly showing a trend similar to that of the capital cost given by Figure 3 and evaporator surface area shown in Figure 4. The steam demand increased by 215% as flow rate increased from 30 to 100 m3/h.

Clyto Access

Figure 5. Effect of flow rate on steam requirements
Effect of recovered heat

The Energy recovered via heat exchangers is used to raise the temperature of the wastewater stream prior to introducing it into the evaporator. The amount of heat recovered directly affects the steam demand and the size of the heat-exchanging units. The aim would be to optimize the heat recovery process so as to minimize steam requirements (i.e. operating cost) and capital cost. Figure 6 shows the effect of heat recovery, expressed in terms of exit wastewater stream temperature, on the process operating and capital costs. The results show an increase in both capital and operating costs with waste stream temperature. The rise in capital cost was particularly significant for stream exit temperatures above 75 oC. This can be attributed to the sharp increase in the heat exchanging surface area,resulting in expensive heat exchangers. On the other hand, the increase in operating cost with temperature was not as significant. The almost flat trend can be explained by the reduction in steam requirements, which offsets the operating costs associated with the heat exchangers resulting in an almost zero change.

Clyto Access
Figure 6. Effect of heat recovery on the overall process cost at flow rate of 70 m3/h

Figure 7 shows the effect of heat recovery on the process steam demand and heat exchanging surface area. As expected, the results show a decrease in both the heat exchanging surface area and steam requirements with waste stream exit temperature. This is due to the narrowing window of sensible heat, bring water to boil. A drop of 10% and 4% over the temperature range from 65 to 85 oC was observed for steam demand and heat exchanging surface area, respectively. It is worth noting that trends similar to those shown in Figure 6 and Figure 7 were observed at all other flow rates (results not shown) for the effect of heat recovery on the overall cost, steam demand and heat exchanging surface area. The only distinction is that changes in these parameters were more pronounced at higher flow rates and the sensitivity of change reduced with flow rate.

Effect of pH

The solubility of salts in water is dependent mainly on concentration, pH, and temperature. Salts start to precipitate out of solution at lower pH and at low temperatures. At higher temperatures, due to the loss of water the concentration of salt also rises which can lead to precipitation. pH adjustment is crucial to prevent precipitates from forming which can foul equipment. It is important to note that if the pH is too low, the water becomes acidic, causing rapid decay of vessels and pipelines due to corrosion. Simulation results suggested that a pH of 7.7 is necessary to maintain suitable operable conditions.

Clyto Access
Figure 7. Effect of heat recovery on the steam demand and heat exchanging surface area cost at a flow rate of 70 m3/h
Methods

Net present value (NPV) is another measure of projected project profitability that examines the difference between the present value of cash inflows and the present value of cash outflows. Even though project A has a higher initial investment, it is anticipated that the additional savings that will accrue in the future will offset this cost. NPV is calculated using equation (1) as follows:

Clyto Access

This section presents and discusses the economic impact of the proposed wastewater treatment process.The proposed configurationis compared to the based case scenario alternative developing new evaporation pond site. For this purpose, the proposed project was referred to as “project A” and the base case as “project B”. Table 3 gives a summary of the economic analysis results for the main evaluating parameters used in this study.

Table 2: Projects economic evaluation parameters

Parameter*

Project A

Project B

Capital cost

$15,394,000

$ 8,000,000

Operating cost

$ 3,786, 000/yr

$ 100, 000/yr

Expected life

10 yrs

3 yrs

Revenue

$ 831,204

$0 

Interest rate

3 %

3 %

Payback period

18 yrs

n/a

NPV

$1,404,073

-$38,699,434

*all monetary values are in USD

The estimated cost of the proposed water treatment processfor a capacity of 100 m3/hr has an estimated capital cost and operating cost of $15,394,000 and $3,786,000/yr, respectively. The base case project of developing an evaporation pond with a capacity of 1,600,000 m3 has a capital cost of $8,000,000 and operating cost of $100,000/yr. Both projects are compared on the basis of a ten-year useful life. Over the course of ten years, it is estimated that three ponds will be required over the useful life of the project. Figure 8 and Figure 9 shows the cash flows for project A and project B, respectively. For project B, the cost for reclamation by the end of the ten-year useful is estimated to be 5% of the capital cost of the fertilizer plant. Considering this type of investment as mutually exclusive, this translates into $45,000,000 in year ten, which is an opportunity cost if project A were to be implemented [14]. For project A, revenue is based on the offset cost of recovered water that would otherwise have to be purchased from an external source at an estimated cost of $2.47/m3. Project B has no revenue, but requires an additional capital expenditure to reclaim.

Clyto Access
Figure 8: Cash flow diagram for project A
Clyto Access
Figure 9: Cash flow diagram for project B

At an interest of 3%, the NPV of projects A and B were found to be $1,404,073 and -$38,699,434, respectively. Figure 10 shows the sensitivity of the NPV to interest rates for both projects. Results from Figure 10 shows that both projects’ NPV were sensitive to changes in interest rate up to 40%. However, project A exhibited positive NPV values for interest rates above 2% while project B is profitable only at high interest rates (>30%).

Clyto Access
Figure 10: NPV sensitivity to interest rate changes for projects A and B

Figure 11 shows the NPV for project A as a function of fresh water price. The results show an exponential increase in NPV values as the price of water increased. As can be seen from the figure, the NPV values are very sensitive to changes in water price which would significantly alter the cash flow in favor of project A. This implies that the higher the cost of fresh water, we are better of implementing project A to recover as much water as possible and generate an internal return that would otherwise be lost.

Clyto Access
Figure 11: NPV sensitivity to water price changes for project A

A techno-economic assessment has been conducted to evaluate the potential of replacing conventional evaporation ponds,found in a typical fertilizer complex, with an efficient and economical thermal evaporation system. The process design and simulation models were conducted using the commercial simulator SuperPro Designer. Also, a sensitivity analysis was conducted in terms of net present value to investigate the effects of key parameters (interest rate and water price) cost assumptions on the proposed project valuation.The study revealed that the proposed system provides a fast evaporation route that not only significantly cuts down on water use but it also provides economic gain. Such wastewater management and treatment strategy has the capacity of freeing up resources such as land and reclaimed water.The proposed treatment was estimated to have a capital cost of $ 15,394,000 and operating costs of $ 3,786,000/yr. The plant has a capacity of 100m3/h and is able to recover 60% clean water for re-use in plant operations.Extensive evaluation of both options indicates that the proposed project is more favorable in the long run with a net present value of $1,404,073.

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI) and the Clean Energy Technologies Research Institute (CETRi) of the University of Regina. Also, the authors thank the staff of Saskatchewan Minerals and Mosaic Potash for donating raw materials to be used in this study.

[1] Temple and scott associates strategic communications. “Fertilizer report for the Canadian fertilizer industry” April 5, 2013.
[2] “Overview of Potashcorp and Its Industry." N.p., www.potashcorp.com/overview/fertilizer-101/global-development-story/ 31 Aug. 2014. Web. 20 June 2015.
[3] United nations department of economic and social affairs, “population facts” No 2011/2
[4] Mohammad Al-Harahsheh, Marwan Batiha, Sami Kraishan , Habis Al-Zoubi. Precipitation treatment of effluent acidic wastewater from phosphate-containing fertilizer industry: Characterization of solid and liquid products. Separation and purification technology 123 (2014) 190-199.
[5] John F. Bossler, Ronald Travis, Clarke Veach, Douglas M. Spolarich. “Treatment of phosphate fertilizer palnt wastewater in Florida for discharge and re-use purposes”. Simens water technologies corp. June 13, 2009.
[6] Water, water use and water pricing around the world. Canadian water policy backgrounder. September 8, 2011.
[7] Sonu Malhorta, Nerri, Nagpur . “Poly aluminum chloride as an alternative coagulant”. Affordable water supply and sanitation. 20th WEDC conference. 1994.
[8] J. C. Copplestone, C. M. Kirk, S. L. Death, N. G. Betteridge, S. M. Fellows. “Ammonia and urea production”. NZ institute of chemistry. 2002.
[9] Fertilizer and Crop Production”. Chemistry and Energy Efficiency. http://chemistry.need.org/content.asp?contentid=147 N.P., n.d. Web 20. June 2015.
[10] N.Qamar, T. Johnston. “Storm pond data 1999 to 2006 report” Yara Belle plaine, 2015
[11] Effluent and solid waste control proposed objectives, British Columbia ministry of environment and parks. Stanley associates engineering. August 1988.
[12] Best available techniques for pollution prevention and control in the European fertilizer industry. European fertilizer manufactures association. 2000.
[13] Prasad Valupadas, Wastewter management review for the fertilizer manufacturing sector. Alberta environmental sciences division. June 1999
[14] Chan S. Park, Ming J. Zuo, Roonald Pelot. “Contemporary engineering economics: A Canadian perspective.” 3rd edition, Pearson Canada, 2012.

Published: 30 March 2017

Reviewed By : Dr. Elsaied Mohamed Abdel Whahed.

Copyright:

Copyright: © 2017 Hussameldin Ibrahim. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.