Treatment and Energy Recovery for Municipal Waste Water Using Forward Osmosis Process

Treatment and Energy Recovery for Municipal Waste Water Using Forward Osmosis Process 

Table of Contents

Abstract

TREATMENT AND ENERGY RECOVERY FOR MUNICIPAL WASTE WATER    USING FORWARD OSMOSIS PROCESS

Background and motivation

Method

Future Work

References

Footnotes

Tables

Figures

Abstract

Treatment of wastewater has become necessary and treatment using membrane is flourishing widely. Forward osmosis process is one such process. The main aim of this study is to investigate wastewater treatment system using forward osmosis membrane. The research reveal a proof study on the feasibility of forward osmosis membrane in two ways: (1) to treat municipal wastewater and (2) to concentrate wastewater for energy recovery using algae biomass. The results will demonstrate how wastewater/seawater forward osmosis system would operate and show the operational cost.

Keywords:  Wastewater, forward osmosis, treatment, energy

Treatment and Energy Recovery for Municipal Waste Water Using Forward Osmosis Process 

Introduction

Modernization is the reason for both – development as well as environmental problems. Swelling numbers of effluents in wastewater has become a global issue. Handful of freshwater resources are not enough to feed increasing population demand. Thus need for wastewater treatment has become necessary. Conventional wastewater treatment plants are designed for treating household and industrial wastewater and to protect environmental from its adverse effect.

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Nowadays many treatment techniques like adsorption, activated sludge, ultrafiltration, coagulation have been applied to treat the wastewater. Membrane separation is an innovative way of treating wastewater. Forward osmosis membrane helps to concentrate suspended solids and nutrients in wastewater. Osmosis is a process in which water passes through the semipermeable membrane to balance the solute concentration. Using FO membrane to treat municipal wastewater is an innovative way which could help to achieve the same results as that of tertiary and advanced treatment (McGinnis and Elimelech 2007; Cornelissen 2008; Achilli 2009). Moreover, FO is a suggested method for algae biomass production and treat wastewater simultaneously if incorporated together (Buckwalter 2013; Wang 2016). Algae produce more energy per area and use less water than terrestrial crops; but, the infrastructure necessary to produce and harvest microalgae is still expensive (Demirbas 2011).

Objective

The objective of this study is to determine the feasibility of forward osmosis process into wastewater treatment plant and harvest algae at the same time. This study proposes a treatment process to harvest the algae with forward osmosis membrane to both create a byproduct revenue and reduce environmental problem caused due to effluents in wastewater. The osmosis system will harvest the algae by utilizing the osmotic gradient between seawater and wastewater. 

Background and motivation

Municipal Wastewater Treatment

Every day, millions of cubic meter of wastewater from homes, institutions, commercial and industries is flushed into the sewer system. Municipal wastewater contain sanitary sewage and sometimes it even contain stormwater. Municipal wastewater is one of the largest source of pollution in water in Canada (Government of Canada).

Figure:1 Water treatment level 1983 – 2009.

(Adopted from Municipal wastewater treatment- Canada)

The operating condition and methodology depends on the level of effluents. Pretreatment, Primary treatment, secondary treatment, tertiary treatment and haulage are the main treatment process followed by the wastewater treatment plants. The percentage of Canadians on municipal sewers with secondary treatment has improved from 40% in 1983 to 69% in 2009, which is approximately 18% done by primary treatment and 13 % by household septic tanks (Government of Canada). Pretreatment removes solid particles while primary treatment reduces the amount of organic solids and inorganic solids.

Compared to these process the secondary treatment and advanced treatment methods require more energy as they have to remove phosphorous, nitrogen and organic matter up to certain extent. Biological treatment is a part of secondary treatment in which dissolved oxygen is added to  remove organic impurities by removing impurities and disinfecting them. Usually, secondary process are designed in such a way that it remove 20-35% 0f nitrogen and 80-95% of BOD₅ from wastewater.

Tertiary and advanced treatment methods remove the remaining suspended solids and pathogenic microorganisms.

Forward osmosis

Various methods are being studied for the treatment of wastewater; out of which osmotic membrane technology is being researched to produce  energy and desalinate water. The phenomenon of forward osmosis occur when water begin to move from feed solution to draw solution to reach osmotic equilibrium. Here the  municipal wastewater is termed as feed solution and seawater as draw solution. Feed solution has low osmotic pressure while the draw solution has high pressure. FO do not require any hydraulic pressure as an input energy, instead it work on osmotic pressure of the solution which create osmotic gradient. The membrane separates the two solution and behaves as a physical barrier for suspended solids and many salts. The figure below shows a the flow of the forward osmosis process.

Figure 2: Flow of forward osmosis process

Forward Osmosis Theory

Assume that the following equation completely rejects the feed and draw solute and water transport through a FO membrane is generally described as:

                         J_{W =} A(_D,b  - _F,b)

Where,

              J_w  is the water flux

             A is the pure water permeability of the membrane

             _D,b  is the bulk osmotic pressure of draw solution and,

              _F,b is the bulk osmotic pressure of the feed solution.

The bulk osmotic pressure is given by Van’t Hoff  equation for both the solution:

                   = R_g T  i M

  Where,

            R_g is a gas constant

T is the temperature in Kelvin

I is the Van’t Hoff factor for specific ion

M is the molarity of the specific ion.

Concentration polarization

As water passes through the membrane, the surface of the membrane is blocked by the solute facing the feed solution which cause the pressure on the surface of the feed solution to be larger compared to the osmotic pressure of the bulk feed solution. This phenomenon is termed as concentrative external concentration polarization for a dense symmetric membrane ( McCutcheon and Elimelech 2007). It is given by

                     J_w = A( _D,b – _F,b exp (J_w,e/ K_f))

The exponential term is the concentrative ECP modulus and is the function of J_w,e .

J_w,e  is water flux and K_f is the mass transfer coefficient on the feed side of the membrane.

On the draw solution side of the membrane dilutive ECP takes place. The water that passed through the membrane weakens the effective draw solution osmotic pressure at the membrane surface. The equation for the combination of both the phenomena is given as under:

             J_w = A(_D,b exp (-J_w,e/ K_D) – _F,b exp (J_w,e/ K_F))

Where,

         K_D is the mass transfer coefficient on the draw solution side of the membrane.

 The negative sign  at the dilutive ECP modulus indicates the reduction of the osmotic pressure on the draw solution side. The above equation is for dense symmetric membrane while FO system works effective using asymmetric membrane i.e., porous support layer and dense layer. However, in FO the concentration inside the membrane has larger effect on water flux.

Internal concentration polarization (ICP) is caused when the salts get accommodated in the porous support layer. The draw solution is in contact with the porous layer. The salt which is on porous layer should allow  to generate the osmotic driving force. Once it is generated the water will pass through the membrane and as a result it will decrease the concentration of the draw solution. The dilutive effect in the membrane is referred to as dilutive ICP and is given as:

                    J_w = A(_D,b exp (-J_w,e K_D) – _F,b exp (J_w,e/ K_F))

Where,

              exp (-J_w,e K_D) is the dilutive ICP modulus and,

              k is the resistance to the diffusion by the solute

Solute Transport

In ideal case of forward osmosis membrane completely block the way of salts; but with current FO reverse salt diffusion is going to happen. Due to this, it is increases the risk to the economy of industrial FO system. If NaCl transfer across the membrane it will be against the strict water quality standards.  Furthermore, the bidirectional transfer of the solutes must be considered. For an example, the transfer of solutes like pathogens and other organic compound from wastewater treatment  to treated salt water may not meet the water quality standards. An equation to predict the amount of reverse salt diffusion is given by (Philip and Yong 2010):

                        J_w / J_s = A/B nR_gT

Where,

J_s  is the total draw solute flux

            B is the draw solute permeability coefficient

            N is the number of dissolved species

Fouling concerns

Membrane fouling is a general problem in FO. It occur mainly due to two reasons (1) organisms use the membrane to attach themselves to (2) due to hydrodynamic force of an osmosis system foultants are drawn into membrane. It decreases the speed of flow rate across the membrane. There are several ways to reduce the effect of fouling like backflushing, increasing the cross-section area, pretreatment and chemical cleaning.

Recent Developments and advantages

Treating complex water like municipal wastewater using forward osmosis membrane has increased over 12 years (Cath 2006; Achilli 2009; Buckwalter 2013 and Ansari 2017). Many of the significant discoveries related to the wastewater treatment using FO membrane and algae separation as described below.

(1)   No need for pre-treatment

(2)   Low energy required due to lack of hydraulic pressure needed.

(3)   As the pore radius is between 0.25-0.37 nm, there is high rejection of salt, pathogens and TDS.

(4)   The process holds excellent working records in terms of durability and water quality.

(5)   The process is easy to apply and very flexible.

(6)   Membrane replacement rates are less

Disadvantages

(1)   Low water fluxes and reverse salt diffusion

(2)   Incomplete rejection of organic contaminants

(3)   Low water flux

Bioenergy from algae

Microalgae have a simple cellular structure. They are the plants which do not have roots, stems and leaves containing chlorophyll. Algae carry out photosynthesis by using solar system to split water and fix carbon dioxide which gives oxygen and storable chemical energy as output. Microalgae have the potential to decrease the water footprint of biofuel production.

Figure 3: Comparison of microalgae’s footprint to other crops

(adapted from Yang, 2011)

According to new findings algae biofuel could compete with conventional energy production if wastewater is utilized as water resource. Anaerobic digestion is a biological process used to convert biodegradable materials into methane and carbon dioxide. Compared to normal electricity production costs, anaerobic digestion of microalgae using wastewater produce energy at high cost.

During the process, the organic material is broken down into insoluble organic polymers. Acidogenic bacteria break the sugars and amino acids into carbon dioxide, ammonia, hydrogen and organic acids. Acidogenic bacteria further breaks organic acids into ammonia and carbon dioxide. Lastly, methanogens convert the remaining products into carbon dioxide and methane that can be used for bioenergy production.

Bioenergy produced from wastewater has two benefits. (1) Sufficient concentration of nitrogen, phosphorous, carbon, etc. are present in the abundant amount in wastewater.

(2) There is no need to construct a new infrastructure to transport wastewater to centralized wastewater treatment plant.

The only drawback of this process is to use algae to supply oxygen and remove nutrients is the removal. According to a study 20-40% of the total algae production cost is due to separation of algae from its aqueous environment ( Grima 2003; Pragya, Sahoo 2013). Harvesting microalgae by FO might be less expensive than other methods, only if the leakage through membrane is controlled and seawater is readily available (Buckwalter 2013).

Method

This section describes the source of data, the necessary assumptions and the economic theory to study the feasibility of the process economically.

Forward osmosis system

Forward osmosis process have following components: a housing structure, feed pumps, storage tanks, flush pumps, FO membrane, valves, piping structure, hangers, FO instrumentation and control system. The FO system was designed based on the advanced treatment system using the reverse osmosis for the wastewater treatment (AWP 2013). The working of the system is shown below by flow diagram.

Wastewater is termed as feed solution and the seawater is termed as a draw solution. Both the solution enter the FO system and are stored in a tank so that the flow is gravitational flow. To prevent both the solutions sulfuric acid and antiscalant is added to them. Feed pumps are used to pump both solutions across the forward osmosis membranes. We get diluted seawater or a concentrated algae when both the solution exit the membrane. Once a month, the flush pumps are used to remove fouling from the membrane. The figure below shows the flow diagram of forward osmosis system.

Figure 4: Flow diagram of forward osmosis process.

(Adapted from thesis by Patrick Buckwalter, 2017)

Assumptions, parameters and cost

System operation parameters

Forward osmosis operating inputs

Capital Cost assumption

Annual Operation Cost Assumptions

Economic rate assumption

All the above tables are taken from the thesis presented by Patrick Buckwalter.

Cost analysis of FO system

The main assumption in the design is that it is similar to the design of reverse osmosis process. The components which are necessary in RO for hydraulic pressure are omitted. System design, operational costs and membrane componentry are proposed based on advanced water purification facility study report (AWPFSR) using reverse osmosis process (AWP 2013).

FO membrane cost is taken as $1500 per 27 m² (Linares 2016).

  Cost of the membrane (FO _cost) = A_FO  C_m

  Where, A_FO is the area of membrane

              C_m is the cost of membrane per square meter.

Membrane area can be given as:

                          A_FO = Q_FS / J_w

Where, Q_FS  is the membrane permeation rate of feed solution

              J_w is the water flux through the membrane.

Costs associated with the pumping and pretreatment were taken double the cost of reverse osmosis process as the FO system require two flow streams across the membrane.

Cost of construction, engineering, permitting, legal, Construction management  was taken from a report by Lundquist, 2010.

                                                     Results and discussion

Initial capital cost of the FO system was about $3.2 million dollars per million gallons of capacity. It is broken down into various compartments. Membrane cost is the largest cost contributor with $2,000,000 for 32,000 m² of membrane. Indirect costs include engineering, construction, legal and administration.

Figure 5: Capital cost components for forward osmosis system (Buckwalter, 2017)

Annual operational cost for the system is about $600,000 per one million gallons of the capacity per year. Osmosis replacement cost is largest contributor to annual operations cost. Compared to other technologies like microfiltration, activated carbon; FO  provides an estimated cost of construction, operation and maintenance.

Figure 6: Operational cost break down of forward osmosis system (Buckwalter, 2017)

Conclusion

Forward osmosis process is not only an advanced wastewater treatment technology but it can simultaneously harvest algae for biofuel and reduce the volume of the wastewater. The FO facility was found to have a lifecycle cost of approximately $10 million/MGD of feed solution.

All in all, forward osmosis concept using feed and draw solution can be considered as a future wastewater treatment concept.

Future Work

Following is the future work to be carried out for this system:

(1)   To remove the total nitrogen demand for large wastewater treatment plants, the solute rejection of ammonium should be rejected and for that membrane improvement becomes necessary.

(2)   Mechanical pre-treatment for the FO process should be investigated  for its operational and economic advantages and disadvantages.

References

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• Ansari, Ashley J, William E Price, Jorg E Drewes, and Long D Nghiem. 2017
• ‘Forward osmosis as a platform for resource recovery from municipal wastewater’ Journal of membrane science.
• Advanced  Water Purification Facility Study Report (AWP), San Diego Water Purification Demonstration Project. 2013. ‘ Indirect Potable Reuse/ Reservoir Augmentation Demonstration Project Advanced Water Purification Facility.’
• Buckwalter, Patrick; Shrewin Gormly, and Jonathan D Trent. 2013. ‘ Dewatering microalgae by forward osmosis’, Desalination.’
• Buckwalter, Patrick. 2017. ‘Forward osmosis for Wastewater Treatment and energy recovery.’
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