Research Article

Feasibility Study of Power Generation from Waste Water Treatment with Patchy Data

John P.T. Mo1,*, Emosi Koroitamana2
1RMIT University, Australia
2Fiji National University
*Corresponding author:

John P.T. Mo, RMIT University, Australia, Email: john.mo@rmit.edu.au

Keywords:

Biogas, Power generation, Waste water treatment, Fossil fuels

It is well known that the treatment of waste water produces biogas as a by-product that is harmful to the environment. The Kinoya waste water treatment plan has been flaring the biogas to eliminate the risk to the society. A feasibility study was carried out to study the possibility of converting the biogas energy to useful power using technologies such as a gas turbine. However, due to historical reasons, it is often impossible to obtain necessary data of the waste water treatment plant to support system design. This paper focuses on estimating the available energy capacity and associated means of disseminating the energy either through the plant’s own power requirements or to be sold to the power grid.

Modern societies generate large volumes of wastewater from both domestic and commercial sources. Wastewater should be treated before discharge to the environment to protect public health and the environment. For a rural community, due to isolation and lack of infrastructure support, water treatment was legislated to be provided on the individual facility [1]. In this situation, the effluent quality and treatment performance should be assessed and regular maintenance of the system should be arranged. For community based wastewater treatment systems, more sophisticated analysis tools including linear programming should be used to optimise system cost and rate of discharge [2].

It is well known that the treatment of wastewater produces biogas as a by-product. The gas is rich in methane due to anaerobic digestion of organic matters [3]. This type of emission is harmful to the environment. Research in pollution management suggested that most emissions were related to human consumption [4]. Syri et al. [5] presented a model to study the impacts of climate change mitigation strategies on air pollution. Amon et al. [6] analysed a specific source of pollution from farmland and found that greenhouse gas emissions were very high and ecologically harmful. However, information for identifying and quantifying the pollution due to emissions was largely lacking. Plans to manage pollution due to wastewater have been largely ad hoc, particularly in handling biogas produced from wastewater.

Many wastewater treatment plants are flaring the biogas to eliminate the risk of gas pollution to the society. However, flaring was found to be one of the sources of increasing pollutants in the atmosphere because of production of carbon dioxide and other heat absorbing gasses [7]. Routine flaring with incomplete combustion generates pollutants which is a threat to human and animal health. Similar study by Edino et al. [8] also showed that political tension and economic adversity were prevalent in areas where flaring took place regularly. Soltanieha et al. [9] suggested that regulatory requirements should be in place for managing the impacts of gas flaring to the environment. Unfortunately, the impacts of flaring were difficult to study due to changing environmental conditions such as wind direction and site locations [10].

On the other hand, biogas is regarded as a form of renewable energy that does not rely on fossil fuels. However, the process of converting biogas energy to useful power is not straightforward. Due to the massive investment requirements, it is necessary to carry out a comprehensive design evaluation study to identify all issues prior to actually putting the idea to practice. A formal design framework is required to guide the design process. Small scale decentralized wastewater treatment are good for small and rural communities. An integrated design methodology based on mass balance was used [11]. The methodology was embedded in a design tool that allowed analysis of different components with site conditions to produce a design satisfying the treatment goals. For large scale development, Liu et al. [12] presented an overview of an engineering framework for planning and design of energy systems. This framework was generic and would depend on the availability of data to support design evaluation. Svanes et al. [13] used a holistic design methodology that incorporated a number of indicators and integrated several evaluation methods. The methodology was regarded as resource and data intensive.

In practice, data scarcity has been a major scientific challenge for accuracy and precision assessment of environmental systems, especially in climate stressed developing countries [14]. It is often impossible to obtain necessary data of the wastewater treatment plant to support system design. To overcome data scarcity issues, Beykikhoshk et al. [15] proposed an approach for targeted knowledge exploration on Twitter. Shabbir et al. [16] relied on statistical extrapolation to estimate the total power demand at peak hour. Gampe et al. [17] used a remote sensing technique to complement data scarce areas. These methods require extensive infrastructure support. This paper uses a systems engineering approach to develop the system design model that can manage data scarce components and focus on estimating the available energy capacity and associated means of disseminating the energy either through the plant’s own power requirements or to be sold to the power grid.

Sludge is a kind of biomass material that is deposited from the treatment of wastewater. From the sludge, biogas is generated from breaking down sludge, organic matter using anaerobic digestion process. The biogas is rich in methane and has the potential for power generation. Unlike solar photovoltaic cells or wind power generators, biomass power generation technology is significantly less dependent on the weather conditions [18]. However, due to its toxicity and corrosive content, the biogas cannot be used immediately in power engines. This literature review, therefore, tries to understand the complete path of wastewater treatment to power generation and possible options. Based on the categories knowledge, a theoretical framework can be developed to guide the design process.

Production of biogas

In activated sludge systems, anaerobic digestion of the sludge produces biogas. The calorific value is defined by the amount of energy produced by the complete combustion of a standard amount of material. The calorific value of biogas from wastewater depends on the inflow wastewater and effectiveness of the fermentation process. Typically, biogas has a calorific value between 21-23 MJ/m3 as compared to natural gas, which has a calorific value of 39 MJ/m3 [19].

The quality of the sludge going into the anaerobic digester determines the quality of gas produced.  Activated sludge or return sludge is most often used in large wastewater treatment plants in situations where the effluent should be of high quality and there is limited space [20]. There are five major groups of microorganisms generally found in the aeration basin of the activated sludge process [21]. The anaerobic digestion by bacteria and algae produces methane and other gasses which can be collected for generating heat.

The typical activated sludge process incorporates mixing screened water with the recycled mixture from the secondary clarifier tank, due to its high content of organisms. The mixture produced is called mixed liquor. The return sludge is taken from the clarifier and returned back into the aeration tank where the mixture is injected with air provide oxygen and causing the solids to remain suspended.

Quality of biogas

Biogas produced in an ideal anaerobic digester could contain 80% methane, which is the result of utilising methanogenic bacteria [22]. However, the quality of the biogas is highly dependent on the quality of the residual water and biological digesters (bacteria). At the end of digestion, both biogas and a digested moist solids are formed which is usually dewatered to produce compost.

In a typical anaerobic digestion facility, the gas composition is about 15% of the total output stream and the liquid and solid components of the residual compost share the remaining 85% in an equal manner [23]. The gas itself, however mainly consists of methane and CO2. The lower amount of methane in the biogas compared to the natural gas (90-95% and 55-65% respectively) classifies biogas as the “low-grade natural gas” [24].

The anaerobic sludge digestion process produces methane gas that has heat value and capacity with the potential to generate power, at an unknown capacity. The overall recovery of energy is a combination of factors such as the effectiveness of the digestion process, the efficiency of energy recovery and the type of treatment process. Table 1 shows the range of gas quality measured by a research team in Switzerland studying two sewage digestion plants over a few months [25].   The amount of gas produced for waste water treatment generally ranges from 0.8 – 1.1 m3/kg of volatile solids destroyed.

 

Max

Min

Average

Methane %

65.22

60.1

62.835

Carbon dioxide %

37.43

32.1

35.11

Nitrogen + Oxygen %

1.3

0.1

0.59

Water (steam) %

3

0

1.685

Hydrogen Sulphide (ppm)

3.77

0

1.47


Table 1. Sewage gas composition.

A study in Bangladesh shows the average biogas to electricity conversion rate [26].   Each cubic meter (m3) of biogas with average sewage gas composition as shown in Table 1 contains the equivalence of 6 kWHr of calorific energy.  When biogas is converted to electricity, in a biogas powered electric generator, about 2 kWHr of useable electricity will be generated, and the rest of the original energy content will be converted into heat which is either disposed of through the cooling system or can also be used for other lower temperature heating applications.

Parameters in an anaerobic digester

Some of the parameters affecting the anaerobic digester include pH, temperature, carbon/nitrogen (C/N) ratio, retention time. These factors need to be kept within a desirable range. A lower C/N ratio cause’s ammonia build-up and ph value exceeding 8.5 which is toxic for the methanogenic bacteria. According to Balat and Balat [27], pH level within digestion system is from 5.5 to 8.5, but methanogens function only in pH range 6.7 and 7.4. A decline in pH would suggest an acid build up and digester instability. To maintain optimal C/N ratio the intake must remain in desirable range. In an anaerobic digester, microorganisms utilize carbon 25-35 times faster than nitrogen; a high C/N ratio indicates a rapid consumption of nitrogen by methanogenic bacteria, resulting in a lower gas production.

Retention time is the amount of time the sludge remains inside the anaerobic digester. Minimum solids retention times are in the range of 2-6 days depending on the temperature for anaerobic digestion system usually, Retention time ranges from 30-60 days [28] The retention time is determined by the average time it takes to break down organic material, this is measured by the amount of Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) of the existing effluent.

Purification Processes

It is clear from the foregoing review that biogas from waste water has a lot of unwanted substances that reduce the system efficiency and cause corrosion. A purification process is required to pre-condition the biogas if it is used in power engines.

Water Scrubbing Technology : Compressed raw biogas is fed into a scrubbing tower where the physical absorption of CO2 and H2S in water occurs at high pressure. The dissolved H2S and CO2 is then collected at the bottom of the tower [29]. This is considered the easiest and cheapest purification method.

Chemical Absorption process : High concentrations of hydrogen sulphide H2S is corrosive to reactors and engines when producing electricity from biogas. Complete elimination of H2S is necessary for robustness of biogas fuelled engines. Reduction of H2S in biogas can be done by methods such as oxidation, external chemical treatment.

Chemical Absorption is usually an extension of the water scrubbing process. During chemical absorption, around 97-99% of the H2S content is removed and leaves most of the CO2 for the water scrubbing to process. The H2S removed by sodium hydroxide (NaOH) washing physical solvents consume less energy so hence are more energy efficient, making it the most commonly used chemical. Different types of physical absorbents are methanol, selexol, rectisol, genosorb, morphysorb [30]. If a small amount of oxygen is present then one column can operate but loading is limited. In some cases, a two column system is implemented to combat this limitation generally

Pressure-Swing Absorption process : Pressure swing absorption process uses the technology of separating certain gasses from other gasses under pressure. Small scale plants with a flow rate of 10m3/hr of biogas are currently operational, this technology is still feasible to be applied to flow rates of 10000m3/hr for biogas. Initial the raw biogas is compressed to 4-10 bar which is then fed to a vassal containing an adsorbent material to retain the CO2. Adsorbent materials are commonly are Zeolites, activated carbon and molecular sieves are used as a trap, in absorbing at high temperature. The enriched CH₄ is collected at the top of the vessel while the adsorbent becomes saturated with CO2.  The H2S adsorbed into the material is usually irreversible so a system should be placed before the PSA to remove H2S [31].

Membrane Purification process : Membrane purification process for gas cleaning utilizes a membrane-porous material that let some gasses permeate through its structure, while other are retained. The most common materials are a hollow fiber made from different polymers. Gasses impurities with small molecular size, low affinity, and a low permeability can be retained.  Impurities such as CO2, O2, and H2O pass through the membrane as permeate, while low permeable CH₄ is retained and collected at the end of the hollow column. The gas is forced at pressure through the membrane structure. Impurities are being drawn out by the membrane leaving only the desired purified biogas [32].

Siloxane removal : Siloxane is organo-silicons added in personal care products. Over 1 million tonnes of siloxane is produced annually by the personal care industry worldwide. This substance is not eliminated in the water treatment process or digesting process and remains in biogas. Siloxanes in biogas can greatly reduce the efficiency of energy recovery from biogas [33]. During combustion, siloxanes are converted into silicon dioxide deposits, leading to abrasion of engine parts or the build-up of layers that inhibit essential heat conduction or lubrication.  The technology from the removal of siloxane includes absorption (fixed bed and fluidised bed), gas and deep chilling, biological removal, catalytic process, membrane technology and in-engine removal approach [34].

Biogas flaring characteristics

Biogas flaring is used to safely burn excess biogas being produced. When designing an appropriate flare the aspects of air requirement, exhaust gas flow rate, stack exit velocity, residence time and energy balance are considered [35]. The temperature of the flare is determined by the amount of air added to the biogas. The relationship of excess air added and the flame temperature is dependent on the heat released from combusting the methane [36].

Methane auto ignites at temperature 537.2oC. The amount of oxygen needed to oxidise methane has been defined as lower explosive level (LEL) and upper explosive limit (UEL). These limits are known as flammability limits. The LEL for methane is 5.0% and the UEL is 15% [37]. This means a volume of air with a concentration between the specified limits will be flammable.

Methods of power generation from biogas

Biogas is about 20% lighter than air and has an ignition temperature range of 650 to 750 degrees. It is also a colourless and odourless gas that produces a clear blue flame when burned similar to natural gas. Flaring of biogas produces a lot of heat. If a power generation system is available, the biogas can be captured and converted to a useful form of energy. Biogas can potentially be used in many types of equipment as methods of electricity and/or heat production.

Production of heat and stream : A basic application for biogas is thermal energy (heat). Small biogas systems can provide enough energy for cooking and heating water. Conventional gas burners with a simple adjustment of the air-to-gas ratio can adjust to use biogas. The quality of biogas needed for a gas burner is low, and only requiring a pressure of 8-25mbar and maintaining H2S levels beneath 100ppm.

Electricity Generation or Combined Heat and Power (CHP) : Combined heat and power systems produce primarily electricity and use the inevitable waste heat for other purposes. The systems of producing both electricity and heat together have an overall combined efficiency greater than just producing either power or heat [38]. Internal combustion engines are most commonly used for CHP production. Gas turbines can also produce both CHP with a comparable efficiency of a spark ignition engine and also have low maintained.

Internal combustion engines : Internal combustion engines require a very clean fuel, otherwise, engine wear and low power output will occur. Methane is the most valuable component for using biogas as a fuel. Other components that don’t contribute to the calorific heating value of biogas are usually removed during the purification system [39].

Internal combustion engines use a higher compression of fuel to ignite the fuel rather than using spark plugs, i.e. using Otto cycle. Otto engines can be operated on biogas. They can also work with gasses with a low mass flow rate [40]. Usually, a small amount of petrol (gasoline) is used to start the engine. An Otto cycle engine producing 18kW of power demands around 5.6m3 of biogas an hour. Biogas-powered engines based on Otto principles require biogas higher than 45% methane content. The electric efficiency varies from 34% to 45%. The duel fuel engine (diesel and biogas) works with a lower electrical efficiency of between 30% and 45%.

Gas turbine : The US tends to use biogas in Gas turbines, more often than other countries. Biogas gas turbines are rarely used in developing countries or for small-scale applications due to its high price. Small biogas power turbines produce a power output of approximately 30-75kW and are readily available to be purchase. The maintenance performed on gas turbines also requires specialised skills. The quality of gas entering the system must be purified as a harmful gas or a high hydrogen sulphide level will shorten the lifespan of the engine and cause significant damage.

Stirling engine : Atkins et al [41] developed a Stirling engine which converted waste wood to power. The engine was designed with air-charged and self-pressurizing to make it efficient for its size and cost. In a Stirling engine biogas is combusted externally, thus heating the Stirling motor through the heat exchanger. Stirling engine has a heavy tolerance of fuel composition and quality. They are expensive and characterised by low efficiency, consequently limiting the application of use.

Direct combustion steam turbine : steam turbine technology system consists of a boiler or heat exchanger which is used to generate steam at a temperature above boiling point and at a given pressure. The steam produced passes through a multistage steam turbine which in turn drives an electrical generator. In the case of biomass utilization biomass is burned using direct combustion system in a combustor or furnace to generate a hot gas, which is fed into a boiler to generate steam, which is further expanded through a steam turbine or steam engine to produce mechanical or electrical energy. Steam turbines follow a thermodynamic cycle known as Rankine cycle. This cycle converts heat to work externally via a closed loop, generally using water a working fuel.

Biogas fuel cells : Fuel cell technology offers the potential to converted Biogas directly into electricity, some facilities demonstrating this are now operational in Europe and North America. Fuel cells are expensive and the process requires a very clean [42]. A fuel cell located in Renton, WA consumes 4360 m3 of biogas and produces approximately 1MW per day. It is being used to power the plant but is equivalent to powering 1,000 households. Biogas to fuel cell boosts net output of electricity by a minimum 60%, compared to reciprocating engines at 30% and turbine engines at 40%. Fuel cells also have lower carbon emission per unit of electricity.

Since the investment for a power generation system is high, it is important to follow a well thought through methodology. The aim is to pursue a proven pathway that helps to systematically research, collect, generate, analyse and apply the data that can be made available in the course of the system’s design. Figure 1 shows the research methodology structure.

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Figure 1. System design methodology.

In Figure 1, it is assumed that there are a few possible routes to success, i.e. provide a blueprint for a biogas power generation plant. Irrespective of the routes to system design, the system design methodology provides a guide to the essential elements and processes that will give the highest probability of successful development of the waste water to power generation system.

The system design methodology can be roughly segmented in three sections. The modelling section is located at the top part of Figure 1 where the physical components are modelled mathematically to represent the behavior of the components during work situations. Once data is obtained, the analysis phase consists of several major engineering determinations, including selection, verification, and numerical computation. Verification of the sub-system components is also essential. Ultimately, the project outcome is a set of drawings and design that can be given to contractors to work on.

Kinoya Waste Water Treatment Plant (WWTP) treats incoming sewage using conventional aerobic process including trickling filters, clarifiers and sequential batch reaction process that is meant for producing treated water. These processes allow separation of suspended solids from the water in the form of sludge that is fed to anaerobic digesters for the sludge stabilization. There is a provision of belt filter press on site for dewatering of sludge to produce sludge that contains low moisture content.  The low moisture content sludge can be sold to other applications.

Based on the research methodology in Figure 1, this case study focused on the available data on the plant inflow, process, and the gas flaring capacity.  The data and analysis outcome of this investigation is intended to support a prospective proposal of a system incorporating a proposed gas turbine or applicable power generating equipment.  This proposal can be used as a base to develop a profile of present and future R&D projects of sewage treatment power generation on the Kinoya WWTP.

Inflows

In deciding what data to collect, the research team has spent some time to observe the type of data that could be readily captured. One of the data types was inflows. On 10 June 2015, the plant was monitored on an interval of 15 mins for 24 hrs. The data can be found in Figure 2. The trend of usage was peak during day time. A mean flow of 32.2 MLD (app. 1342.42 m3/hr or 372.9 L/s, Approx. 161,090 EP) was recorded. This data indicates that the measurement was taken on a sunny day.

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Figure 2: Kinoya WWTP inflow observed on 10 June 2015

This gives a benchmark for understanding a realistic possible amount of biogas. To determine the future supply, Figure 3 shows the projected sewer in the next 20 years.

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Figure 3: Kinoya WWTP projected inflow in the next 20 years

The data set in Figure 3 incorporates the information from the master planning unit with liable development on the plant and likewise the growth in sewer demand. Inflows are differentiated into three categories:

• Average dry weather flow (ADWF)
• Peak dry weather flow (PDWF)
• Peak wet weather flow (PWWF)

The infiltration, illegal connection, industrial waste and government waste are factored in this inflows projection. Therefore, the current plant capacity is capable of treating the ADWF and slight overload during wet weather (PDWF) but will not be very effective during PWWF.

Furthermore, the effect of inflow due to rainfall is investigated by checking independent information from the Bureau of Meteorology. From Fiji Meteorological Service [43], the rainfall varies in the year.  Figure 4 shows a typical annual rainfall days’ pattern for Suva and Nadi. The Suva area has more rainy days than Nadi and the pattern seems to be more uniform over the year. From another study jointly conducted by Fiji Meteorological Service [44] and the Australian Commonwealth Scientific and Research Organisation, there is no clear trend of annual or seasonal rainfall at Suva and Nadi Airport from since 1950.

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Figure 4. Number of rainy days in the month for Suva and Nadi.

This information indicates that the increasing inflows in the next 20 years as shown in Figure 2 will be due to population growth rather than affected by the amount of rainfall.

Treatment process

The treatment process of Kinoya WWTP uses a conventional aerobic process including trickling filters, clarifiers which has being upgraded and developed over a decade. These processes allow separation suspended solids from the water in the form of sludge that is fed to anaerobic digesters for the sludge stabilization.

The sewage treatment plants are meant to remove solids and dissolved materials from the incoming sewage. The residuals removed from the unit operations are called sewage sludge that needs further treatment before disposal. This treatment is, in general terms, a type of anaerobic digestion that kills pathogens and reduces the mass of sludge by decomposing organic matter through the breakdown of volatile solids. The anaerobically digested sludge is referred to as bio-solids, which can be applied to amend the soil or disposed of at landfills. The plant has two existing anaerobic sludge digester with one under repair at the time of this study. The gas flaring is found to be conducted for 8 hours daily. The combined volume of the digester is 3,050 m3 and the estimated retention time is 23 days.

Energy content in biogas

From the literature review, it was estimated from other information that about 2 kWHr of electricity form one m3 of biogas can be generated. Information of the biogas over the first half of June has a composition as shown Table 2. The methane content is much lower than the published data and there is a very high hydrogen sulphide content. High hydrogen sulphide content hinders methane production. The data indicates that the bio-digester condition is still not optimum.

Methane (%)

Carbon dioxide (%)

Hydrogen sulphide (ppm)

Oxygen (%)

46.8

41.0

4979.4

12.3


Table 2. Recorded biogas composition in first half of June.

If we take the current flaring rate of 8 hours per day at a flow rate of 257 m3 per hr, the total amount of biogas is 2,056 m3 per day.  This can then be turned into 4,112 kWHr or 4.1 MWHr of useable electricity.

The Bio-chemical Oxidation Demand (BOD) is an indication whether Kinoya WWTP is treating waste effectively or not.  The final effluent BOD should be within the standard range of 40mg/L.  Figure 5 shows the level of effluent over the years 2007 to 2014.  In 2011, data recorded by WAF shows a lot of fluctuation.  It is noted that during this period, WAF has been grading Trickling Filter No 1, restored Primary Clarifier No 1 and Secondary Clarifier No 2 and upgraded sludge pumps.

Since the level of BOD is still to be improved, a number of bio-solids will increase in the future once the treatment processes in Kinoya WWTP are rectified to the as-designed level.

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Figure 5 . BOD of raw sewage & final effluent.

Gas production analysis

According to information from the Operation team, it is believed to be flaring at an average rate of 257 m3/hr. In order to obtain statistical data for analysis, the SCADA system was studied to see if information could be downloaded from the SCADA system’s database (Figure 6).  However, this information has not been made available due to various reasons.  Hence, the gas flaring data collected so far are manually recorded from the display.

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Figure 6: Sample screen of SCADA system

Figure 7 shows the rate of gas flaring on a typical data in June.
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Figure 7. Recorded rate of gas flaring on 1/6/2015

Figure 7 shows the relation between the current flow rate of gas flaring with time on a result batch, the measuring meter captures this result with a time interval of 10 minutes and currently flaring an 8 hours daily.  Over the recorded period, the gas flare flow rate seems to quite steady at 272 m3/hr within 1% variation.  This measurement somehow validates the ability of the system to flare at least the designed capacity of 257 m3/hr.

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Figure 8. Temperature for different gas flaring flow rate

Furthermore, the temperature of the gas flaring was recorded against the immediate gas flaring flow rate during the data logging session. If we assume that the volume of gas remains constant during the combustion in the flare, the temperature can be approximated to some kind of relationship with the gas flaring flowrate. The “regression” line in Figure 8 is inserted to show a possible modelling approach but it is still early to conclude. In addition, it can also be seen that at some instances such as the point at the most right-hand end of the graph, the relation of these deviated points does not exhibit the same pattern. This means there may be an unforeseen reaction that needs to be investigated further. Factors such as the quality of gasses and the control of flow rate can affect the chemical reaction in the flare and are unknown at this stage.

Preliminary Gas Turbine Assessment

The gas turbine is the most unused technology in Fiji in terms of renewable energy. With high electricity expenses, the research team will need to tap on such ingenious way to supplement electricity with the availability of by–product to biogas which is sludge in this instance.

Based on the information collected so far, if a consistent sewage increase is assumed, treatment, biogas production, gas control, after taking into account the loss of power generation from the gas turbine to the electricity generator, the amount of electricity energy available can be seen in Figure 9.  At the current ADWF inflow, the estimated power available is 4.1 MWHr per day, increasing to 9.1 MWHr per day in 2033.  However, the PWWF inflow data shows current power at 9.8 MWHr per day to 26.4 MWHr per day in 2033.  The average power available would lie somewhere around 5 MWHr per day at present to 11 ~ 15 MWHr per day in 2033.

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Figure 9. Projected electricity power available

Preliminary System Design

The research team proposes to carry out a pilot study of a smaller scale gas turbine installation and monitored operations using the biogas produced by the Kinoya Waste Water Treatment Plant. An important part of this proposed research is to set up a comprehensive SCADA system that will form the backbone of future automated Kinoya WWTP.  Due to lack of system information and development options, we are unable to obtain automated logs of data required for analysis. The specific parameters are:

  • • Plant inflow flow meter
  • • Gas flow meter from the Digester to the biogas holder
  • • Gas flow meter from the Gasholder to the flaring furnace
  • • Quality (or composition control) of biogas
  • • Kinoya WWTP operating parameters such as plant load, sewage level fluctuations, etc.

Figure 10 shows a preliminary concept of a biogas power generator and feed system to the Kinoya WWTP.

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Figure 10. Preliminary system concept.

The following operating criteria for the design of the system are required:

  • • 24 hours continuous operation
  • • Gas purification to enhance power efficiency and minimise unexpected impact to the gas turbine
  • • Load monitoring system to balance loading to the gas turbine with grid power if necessary.

The proposed gas turbine will have a capacity of 0.5 MW to 1 MW depending on more accurate data being collected in the initial stage of the proposed R&D project.  This level of power generation will meet the current demand of the Kinoya WWTP and allows scope for scaling up to the future. 

Since the energy content of biogas is estimated at 4.1 MWHr, the gas turbine will only run for a maximum of 4 hours per day.  However, the power demand for the Kinoya WWTP will vary during the day (max. 0.7 MW during peak inflows but will be less at other times), this amount of energy can be spread over a 24 hours period.  Therefore, research on how to operate the gas turbine power at non-peak times to preserve biogas fuel is required.

The treatment of waste water produces biogas as a by-product that is harmful to the environment. This paper reviews literature to define the essential elements in the design of a power generation system from biogas produced from wastewater. Due to historical reasons, many wastewater treatment plants are developed on an ad hoc basis and are unable to provide sufficient data to support large scale system development such as power generation from biogas. A system design methodology is proposed in this paper to form a guide to the design of the wastewater to power generation system in the case of Kinoya wastewater treatment plant. Preliminary information fitting into the system design methodology seems to suggest that a gas turbine power generation system is possible but care should be taken to maintain a close to consistent waste water flow and effectiveness of biogas conversion.

The authors would like to thank the support of Water Authority of Fiji on this research work and in particular, Mr. Opetaia Ravai, for allowing the research team to collect data from the Kinoya wastewater treatment plant in this research.

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Published: 14 April 2017

Reviewed By : Dr. Elsaied Mohamed Abdel Whahed,

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