Introduction
Treatment of domestic wastewater is an integral part of minimizing contamination of the environment, particularly surface and groundwater reservoirs containing raw water to be processed for human use. Most wastewater treatment plants in Trinidad utilise the Activated Sludge (AS) process to remove or reduce major organic pollutants including 5-day Biochemical Oxygen Demand (BOD5), Total Suspended Solids (TSS) and Chemical Oxygen Demand (COD) using biological means. Removal of these pollutants is achieved by addition of sufficient oxygen to the raw wastewater, which oxidises some chemical pollutants and encourages growth of microorganisms to digest the majority of the organic waste. Oxygen is supplied by large air blowers, one of the major consumers of energy in wastewater treatment plants. The other major consumer of energy is the pumping system used to transport raw water to the treatment plant and in some cases, to transport treated effluent from the final treatment process.
The application of renewable energy in wastewater treatment is possible in multiple ways. It can be used directly as in the case of solar radiation providing heat energy for reactions in stabilization ponds or indirectly by using wind turbines and/or photovoltaic cells for electricity generation to power equipment. Some of the major advantages of using renewable energy sources are the low to non-existent levels of pollutants generated from their use and, of course, the potentially endless supply of energy. Some of the disadvantages are the relatively high initial capital investment and the variability of available energy based on location. Many of the wastewater treatment plants in Trinidad are relatively small (treat waste from <20,000 persons) and therefore consume small amounts of energy individually. However, combined, they can use significant amounts of energy. Applying renewable energy as a power source for all plants can significantly reduce the country’s dependence on conventional energy sources and reduce the pollution associated with power generation using these conventional methods.
The aim of this project is to determine the feasibility of using renewable energy as the sole energy source in municipal wastewater treatment plants in Trinidad. The research question for this project is: ‘Is it feasible to use renewable energy to fully power a wastewater treatment plant in Trinidad’?, while the ultimate objectives are as follows: 2
‘ Objective 1: Evaluate the various options for applying renewable energy to wastewater treatment
‘ Objective 2: Compare the financial cost of powering wastewater treatment plants using grid-connected (conventional) power versus renewable energy for wastewater treatment
‘ Objective 3: Compare the environmental effects of powering wastewater treatment plants using grid-connected (conventional) power versus renewable energy for wastewater treatment
Literature Review
The use of renewable energy in wastewater treatment is possible in various forms. It can be used directly as in the case of solar radiation providing heat energy for reactions in stabilization ponds or indirectly by using wind turbines and/or photovoltaic cells for electricity generation to power pumps, ultraviolet (UV) disinfection systems and conventional surface-water treatment systems (Argaw 2001, 1). Renewable energy sources, unlike conventional power sources, are mostly used for small to medium applications because of their high initial investment costs. The power needed to treat a rural water supply is relatively small and for this reason renewable energy power sources are widely used in many developing countries (Argaw 2001, 1). The Photovoltaic Solar Electro-Oxidation (PSEO) process combines the effectiveness of the electrochemical oxidation based on boron-doped anodes to mineralize organic matter, with the autonomy and environmentally friendly characteristics of photovoltaic solar energy (Alvarez-Guerra, Dominguez-Ramos, and Irabien 2011).
Wind machines are not normally used for wastewater treatment, since most wastewater treatment systems have very large power requirements (Argaw 2001, 1). Wind pumps can be classified in 2 categories, mechanical or electrical. A mechanical wind pump or windmill uses a reciprocating pump, positive displacement pump or piston pump located below the water level. The crank of the windmill must exert sufficient force on the pump rod to lift the weight of the pump rods, the piston and the water in the piston, and to overcome the friction (Argaw 2001, 11). The size of the pump determines the starting wind speed; a larger pump requires a larger starting torque, therefore a larger starting wind speed. An average rotor diameter of 7.6m can produce 2kW of power at 3m/s wind speed (Hodgkin, White, and McGowan 1987). Electrical wind 3
pumps produce direct current (DC) or alternating current (AC) and can be connected directly to pumps to supply power. They have the advantage of not having to be placed directly over the pump, allowing these wind machines to be placed in the most optimum location for wind harvesting. However, they require a higher starting speed and perform better at high wind speeds (Argaw 2001, 13).
Solar energy is one of the most promising renewable energies. Unlike conventional generation systems, solar photovoltaic (PV) energy systems generate electricity silently with little maintenance, no direct pollution or depletion of resources (Hrayshat 2009). Modern solar PV systems utilise Silicone solar cells; small semiconductors which absorb photons from sunlight which excites electrons in the cell and allow them to carry an electric current (Lynn 2010, 36). These solar cells are the basic units of PV modules, which in turn are the basic units of a PV array. New PV modules range in size from 2 watts peak power (Wp) per panel to over 240 Wp per panel. Panels with larger power generating capacities and efficiencies minimize the surface area required to generate a given power output.
In today’s society it is of vital environmental and economic interest that energy consumption is minimized and wastewater treatment is a costly business with regard to energy (Rydh and Akesson 2007). An energy usage inventory is a structured way of determining the energy requirements of a wastewater treatment at each stage of the processes and for the plant as a whole. The method of calculating energy requirement for all equipment and structure of the inventory are well developed and can be modified for Wastewater Treatment Plants (WWTP’s) in Trinidad. The use of two axes solar tracking systems will also be proposed as these increase energy output for a given solar array by over 20% (Fernando Cruz-Peragon 2011, 1). This development can lessen the quantity of solar panels required to supply sufficient power, therefore significantly reducing initial capital cost as well as land space required.
Research Design
In this project, the hypothesis to be tested is ‘It is financially and environmentally beneficial to use renewable energy for wastewater treatment over current methods of supplying energy’. To effectively determine the feasibility of using renewable energy in wastewater treatment, the following variables will be investigated: 4
‘ The amount of energy required to achieve wastewater treatment which will comply with legally-required effluent quality
‘ Solar radiation (insolation) and wind speed in the study areas which will determine the potential amount of energy to be generated. This will be monitored using appropriate equipment and data collected for at least one month.
‘ The current environmental and financial costs of generating electricity used for wastewater treatment. This will be done by conducting interviews as well as gathering documented data on liquid, solid and gaseous emissions from power generating plants
‘ The environmental and financial costs of using solar and wind energy for wastewater treatment. Data on this will be gathered from extensive research papers already conducted on this area of study.
Methodology
The proposed research project will be undertaken in 3 phases:
Phase 1 will begin with reviewing literature on topics applicable to the study including but not limited to photovoltaic electricity generation, wind turbine electricity generation and photovoltaic solar electro-oxidation. It will also include determination of the energy requirements for the WWTP’s being studied by analysing the processes and equipment utilised, then compiling an energy usage inventory for each site. The proposed sites to be studied are:
1. The Cashew Gardens Wastewater Treatment Plant
2. The Orangefield Wastewater Treatment Plant
This will be accomplished by conducting interviews of the operators and managers of the WWTP’s and site visits to gather information personally. The potential energy to be generated using solar and wind energy will be determined by using a pyranometer and an anemometer at both sites. Data will be logged for at least one month and then analysed. The gathering of both field and documented data will allow for the achievement of objective 1, evaluating the various options for applying renewable energy to wastewater treatment. The major costs of this phase will be the rental or purchase of a pyranometer and an anemometer with data loggers for both. 5
Equipment from the Faculties of Engineering and Natural Sciences are currently being sourced and no local commercial suppliers of the required equipment were located. Foreign suppliers have been sourced for purchasing the equipment, with the pyranometer at US$210 and an anemometer along with a datalogger from Onset Computer Corporation for approximately US$239 and US$108 respectively (Corporation 2012). A research-grade pyranometer can also be constructed for approximately US$30 (Brooks 2007). The use of data already gathered such as insolation maps and wind speed maps can also serve as an alternative to measuring the wind speed and insolation at the sites.
Phase 2 will include the analysis of data obtained from interviews, solar irradiation and wind speed measurement equipment and environmental effects of both renewable energy use and conventional power generation. The data will be processed by using established models for calculating the energy output from solar photovoltaic, wind turbine, solar thermal and other renewable energy sources. Power generating systems such as photovoltaic arrays and wind turbines will be designed based on the data obtained in phase 1.
Phase 3 will allow for the achievement of objectives 2 and 3, comparing the financial and environmental aspects of using conventional and renewable energy sources for wastewater treatment, respectively. Environmental cost-benefit analyses will be conducted to determine if the financial costs and environmental effects of using renewable energy for wastewater treatment make it a feasible and beneficial option.
Data Collection and Analysis Timetable
The majority of data collection will be done in Phase 1. The most time-consuming aspect of data collection will be monitoring of insolation and wind speed at the proposed sites which will be conducted daily for 30 days. This will data, along with other field and written data, will be analysed in phase 2. Figure 1 below shows an outline of the major aspects of the project and the estimated time required. However, many aspects of the project will be conducted simultaneously, such as additional literature review will be conducted while insolation and wind speed data are gathered at the sites. Solar irradiance measured as kW/m2/day and wind speed measured as m/s will be averaged over the monitoring period. These values will be used to size photovoltaic arrays and wind turbines, as well as determine the energy available for other uses of renewable energy in wastewater treatment at the sites. 7
Conclusions
The main focus of this project is to determine if domestic wastewater treatment can be accomplished while using renewable energy only. The accomplishment of this would reduce the consumption of fossil fuel use as a nation, since wastewater treatment at the many plants throughout Trinidad collectively requires a significant amount of energy.
The expected outcomes of the project are as follows:
1. It is possible to fully power existing wastewater treatment plants in Trinidad using one or more types of renewable energy
2. It is environmentally advantageous to utilise renewable energy for wastewater treatment as far less solid, liquid and gaseous pollution is produced when compared to using power from conventional sources for wastewater treatment
3. The initial financial investment is high when using renewable energy in wastewater treatment, however, savings in power cost as well long-term payment plans for equipment make renewable energy an attractive option. In some cases, it can even be the less costly alternative to using grid-connected power.
4. An environmental cost-benefit analysis will show that it is financially and environmentally beneficial to power wastewater treatment plants in Trinidad using renewable energy. 8
References
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