Energy from Sewage – Renewable energy to be tapped to make environment green:
Sewage treatment, that is, the physical, chemical and biological processes used to clean industrial and domestic wastewater, has improved significantly over the past 20 years. However, the energy requirement to treat sewage to the highest standard has been quite large. Further tightening of water quality standards, especially in developed nations; suggest energy costs will increase substantially in future. We will discuss here about possibility of renewable energy generation from sewage, to offset the extra energy requirement for sewage treatment and also to use surplus energy for domestic purpose. In fact, the actual energy used will depend on the quality of sewage and intensity of treatment required.
A. Typically, there are three stages of treatment:
(a) Primary - Solids are physically settled out.
(b) Secondary - Bacteria convert organic matter to a carbon-rich sludge.
(c) Tertiary - Further treatment may be used to remove more organic matter and/or disinfect the water.
The effluent is generally discharged to fresh, ground or coastal water. Sludge is applied to agricultural land, incinerated, used for land reclamation or used for other purposes, such as composting or landfill etc.
B. In order to increase energy efficiency of water treatment and reuse of treated water few points have been discussed. By implementing these measures energy savings and efficiency of about 40% may be achieved:
(a) Choosing low-energy treatment options, if possible. However, local constraints may limit choice.
(b) Replacing machine parts, such as pumps and motors, with more efficient versions.
(c) Optimizing processes using sensor technology. For example, pumping can be adjusted according to flow.
(d) Reusing water. “Greywater” from bathing, laundry and washing dishes can be reused to flush WCs. This may provide savings of around a third of daily household water demand.
C. Energy generation - There are mature, widely-practiced technologies for generating fuels from sewage treatment. Moreover, research has identified future methods for exploiting sewage as an energy resource as well.
1. Current Technologies for Energy Production -
(a) Sludge Incineration - Most of the sewage sludge produced at sewage treatment plant is applied to agricultural land as a soil conditioner, reducing the need for fertilizer. Sludge may also be incinerated, with the option of energy recovery. However, to incinerate sludge, it must be dry enough to burn with no extra energy input other than that needed to fire up the incinerator. It therefore needs dewatering, using energy intensive processes such as centrifugation or thermal dehydration. Centrifugation requires less energy but surplus heat from incineration that can be used for thermal dehydration. There has been strong opposition from some sections of the public over incineration of wastes due to fears about impacts on human health. At present, reuse of sludge via application to land is generally considered a more acceptable option.
(b) Biogas - Biogas production from sewage sludge treatment, via a process called anaerobic digestion, is already a well established means of generating energy in many developed countries. Bacteria used to organic matter in sludge to produce a mixture of methane (CH4 of 60 – 65%), carbon dioxide (CO2 of 35 – 40%) and trace gases. Impurities, such as hydrogen sulfide and water, are removed and the resulting biogas is then commonly used in boilers or combined heat and power systems. Biogas may also be used for other applications, such as vehicle fuel, if CO2 is also removed. Anaerobic digestion also reduces the solids content of sludge by up to 30%, reducing the energy costs involved in its transport.
2. Future Technologies for Energy Production - There are several novel technologies that produce energy or fuel as a by-product of sewage treatment, although further work is needed to improve performance, reliability and cost-effectiveness.
(a) Conversion of sludge to oil and gas - Under carefully controlled conditions and extreme temperatures (450 – 1000 degree Celsius), sludge may undergo chemical reactions to produce fuels that may be used for energy production. Processes include gasification, which produces syngas (similar to natural gas), and pyrolysis, which produces bio-oil (similar to diesel oil). There is interest in these as potential alternatives to incineration of sludge. However, operational costs are high, particularly those of maintaining high temperatures, and conditions must be carefully controlled to prevent formation of harmful by-products, such as hydrogen cyanide.
(b) Biomass Crops - In some of the European countries, sewage sludge is applied as fertilizer to willow plantations. The trees are periodically coppiced and the wood used for fuel. Research into applying partially-treated, liquid sewage to biomass crops is also underway. Passage of the sewage through the soil acts as a final polishing step for treatment, degrading organic matter, reducing nitrogen and phosphorus and producing a cleaner effluent. Little energy is required and capital and operational costs are low. However, it is not yet known how efficient this system will be at removing pollution and there must be appropriate land available.
(c) Hydrogen from Sewage - There is much interest in hydrogen as a fuel, because it can be produced from a wide range of materials and provides power with minimal air pollution. Bacteria use organic matter to produce hydrogen by fermentation. However, applications for hydrogen, such as fuel cells, are not yet in widespread use.
(d) Microbial Fuel Cells - These devices offer the possibility of simultaneous sewage treatment and energy production, with water, CO2 and inorganic residue as by-products. Bacteria use organic matter to produce electricity. To date, only lab-scale microbial fuel cells have been developed in some of the developed countries that are able to power small devices.
D. Discussion on energy conservation and renewable energy in relation to sewage treatment system –
(a) Energy conservation is possible through the twin practices of efficient water use by consumers and efficient energy use by the water industry.
(b) There are well-established renewable energy options, such as biogas, and novel technologies, such as gasification, for sewage treatment. Many need further investment and research.
(c) Economic and water quality considerations are key drivers for the water industry. Integration of energy related objectives into the existing regulatory framework will be necessary.