Wastewater treatment plants (WWTPs) play a critical role in preserving natural resources by recycling water and protecting surrounding ecosystems from contamination. They are essential for industrial water reuse and help maintain the quality of our water bodies. However, these plants require significant amounts of energy to operate, which has become a growing concern.
As industries strive for greener practices, improving energy efficiency in WWTPs is more important than ever. In the United States alone, wastewater treatment facilities account for up to 2% of the nation’s total electricity consumption. With treatment and maintenance demands on the rise, operational costs are expected to increase by 30% to 40% over the next two decades.
Given these challenges, it is imperative for wastewater treatment plant operators to adopt sustainable energy-saving strategies. In this article, we will explore various innovative solutions aimed at reducing energy consumption and promoting sustainability in WWTP operations.
Although we aim to reduce wastewater consumption by industries and save the planet Earth, if the energy by which these WWTPs operate, originates from fossil fuel incineration, there are huge emissions of greenhouse gases and increase the global warming potential. Furthermore, energy represents a huge portion of the operational expenses of wastewater treatment processes.
Wastewater treatment plants in the United States utilize more than 30 terawatt hours of electricity each year, totalling $2 billion in annual electric costs. Estimates show that electricity expenditures can account for up to 40% of a wastewater treatment plant’s annual operating budget.
Here are the major benefits of reducing energy consumption in WWTPs:
There will be significant cost savings with long-term energy-saving goals. As we mentioned above, the energy consumption is a major expense in operational costs. Thus, if you optimize this, it will help in achieving economic goals.
The electrical energy is generated through the incineration of fossil fuels that emit greenhouse gases. Reducing energy consumption will lead to lower greenhouse gas and air pollutant emissions. Energy-efficient WWTPs not only reduce bills but also protect public health by reducing air pollution.
Efficient technologies also increase the lifespan of the infrastructure and equipment of WWTPs. Furthermore, effective equipment requires less maintenance and can dramatically minimize the number of emergency discharges from treatment plants.
We can divide the methods of minimizing energy or electricity consumption into two major categories. The first approach focuses on the operational modifications applied at different sections of WWTP, while the second method incorporates the innovative process of wastewater treatment with less energy demand vis-à-vis traditional technologies. Let’s discuss each approach in detail:
The operational strategies majorly include upgrades to mechanical equipment and aeration systems because they consume a major portion of energy. Here are the major operational strategies:
The pumping operations consume a lot of energy in WWTPs. Thus, regularly upgrading the pumping motor is essential to reduce energy consumption. As per estimates, electric motors can make up 90% of the electrical energy consumption of mechanical devices. It is always worth paying close attention to pumps and renewing or servicing them if the energy level audits show a significant increase in operational costs.
Variable speed operation is generally the most energy-efficient flow control option for pumping systems, because pump performance may be modified to suit process demand rather than correcting additional hydraulic losses. Variable frequency drives (VFDs) provide a rapid return on investment, with payback times ranging from 6 months to 5 years.
The automated control of the aeration process is a major way to save significant energy by swiftly adapting to changing reactor conditions. The organic and ammonia loadings of the influent determine how much oxygen is required to sustain biological activity on an aeration basis.
Due to this, the demand for oxygen for aeration exhibits the same diurnal pattern. It increases during the morning and evening, while drops during the midnight. Usually, the ratio of peak to minimum oxygen demand is 2:1.
Since less energy is needed for aeration, it is hypothesized that lowering the dissolved oxygen set points can result in significant energy savings and lower greenhouse gas emissions.
The most common aeration systems are controlled based on dissolved oxygen measurements. However, as the ammonium content gets closer to zero, maintaining the DO (dissolved oxygen) levels could lead to needless aeration. Ammonia-based aeration control can be used to mitigate this condition, which can lower peaks in the effluent ammonia concentration as well as aeration expenses.
The Ammonia versus nitrate (AVN) control was developed to achieve nitrite shunt through NOB (Nitrite Oxidizing Bacteria) suppression. But it has more potential than just nitrite shunt and provides more efficient nitrogen removal than ABAC.
By some reactions, it can oxidize only the amount of ammonia that can be denitrified utilizing the influent organic carbon. This method maximizes the COD (Chemical Oxygen Demand) efficiency without the need for the addition of supplemental carbon. This can be achieved with either continuous or intermittent aeration.
The transformation of ammonia into nitrogen gas requires a lot of energy as it has a high oxygen demand for the nitrification process. Its alternative i.e. deammonification or partial nitritation process will require lower energy consumption. The partial nitridation process is a big innovation development in WWTPs in terms of energy saving.
The nitrite shunt process was developed to overcome the high energy consumption challenges associated with the conventional biological nitrogen removal process. This process is also called Shortcut Biological Nitrogen Removal (SBNR).
In this process, the ammonia oxidation step gets terminated at the nitrite stage. This is known as partial nitrification, then nitrite is reduced to nitrogen gas. SBNR relies on the direct conversion of nitrite generated in the first step of nitrification to nitrogen gas rather than oxidizing nitrite to nitrate since nitrite is an intermediate component in both nitrification and denitrification. Here are the major benefits of the SBNR process:
The autotrophic deammonification process is highly economical for treating ammonia-rich wastewater. It requires no organic carbon source and less than half of the aeration energy vis-à-vis the conventional nitrification-denitrification.
Anaerobic ammonium (anammox) oxidation is an autotrophic process for ammonium removal. It requires less energy but anammox bacteria grow very slowly. The growth rate of anammox bacteria goes maximum to the range of 0.019-0.08 and 0.13-0.14 per day for slow and fast-growing species respectively.
Deammonification is a two-stage process, with the first phase converting half ammonium to nitrite. The second phase is the anammox process, which converts the remaining ammonium to nitrogen gas using the generated nitrite. Both processes are autotrophic and can be conducted in a single-step sequencing batch reactor (SBR) setup.
Deammonification by ammonia-oxidizing bacteria mixed with anammox yields up to 70-90% nitrogen removal with a 65% reduction in aeration energy and a 100% reduction in supplemental carbon when compared to standard nitrification.-denitrification
The wastewater has several nutrients and energy sources that you can recover through WWTPs. We can categorize the recovered energy into three broad categories i.e. chemical, thermal, and hydro energy.
In the wastewater, the calorific energy is the energy content stored mainly in the form of various organic chemicals. As per studies, the energy content in untreated wastewater was estimated to be 10-15 MJ/kg COD. This chemical energy can be converted into biomass energy during biochemical treatment. The recovery of chemical energy involves a transformation of wastewater constituents into gaseous, liquid, or solid fuels.
Here are the different methods of generating chemical energy in different forms:
Biogas generated from sewage sludges are generally composed of methane (60-70%) and carbon dioxide (30-40%), with slight concentrations of nitrogen, hydrogen sulfide, and other constituents.
The methane fraction of biogas is a useful fuel that, with proper conditioning, can substitute natural gas for a wide range of energy requirements. AD is more frequent in plants larger than 22,000 m3/day since larger WWTPs have more accessible sludge for digestion, making it more beneficial for plant managers to consider an AD unit.
Methane has a heating value of about 10 kWh/m3. If we assume a methane content of 65% by volume, the heating value of biogas produced in WWTPs can be approximated to be 6.5 kWh/m3. The most easily adaptable strategy to reduce external energy requirements with current treatment plants is to make full use of CH4 produced by traditional AD through cogeneration in CHP (Combined Heat & Power) systems.
These systems are reasonable if the heat consumers are within the plant’s vicinity. But this is not possible in most of the cases. As a result, the excess heat energy must be vented into the atmosphere, lowering total energy production efficiency and necessitating more electrical power. This energy loss can be avoided by upgrading biogas to natural gas grade. The enhanced biogas, in the form of a natural gas alternative, can be pumped into existing natural gas grids or used as a vehicle fuel via natural gas infrastructure. The enhanced biogas can be provided to users at a cheap cost, allowing them to use it more efficiently.
There are rich organic compounds in the primary sludge of wastewater treatment that can be used for energy recovery. CEPT promotes the primary settling and allows faster coagulation of particles in wastewater. It helps in the faster formation of larger conglomerates and makes the removal process more efficient.
The removal efficiency of primary sludge tanks ranges from 40-60% for TSS and 25-40% for COD. By adding chemicals, these efficiencies can be increased to around 80-90% for TSS and 50-70% for COD removal.
The hydrolysis step limits the conventional AD degradation of organic sludge fraction. It rarely achieves degradation of volatile solids higher than 50%. This restriction stems from the challenge of reaching and breaking down the waste-activated sludge’s (WAS) bacterial cells. By using the mechanical, chemical, thermal, or physical pre-treatment process, this shortcoming can be addressed.
Anaerobic bacteria can use the cell contents released by breaking down the bacterial cells during pre-treatment procedures to produce biogas. Depending on the method, this raises the reduction in volatile solids that is accomplished in AD and, as a result, raises the production of biogas by 20–50%.
The bioelectrochemical systems include the usage of technologies like microbial fuel cells (MFCs) and Microbial Electrolytic Cell (MECs). These are highly promising technologies for energy production from organic matter present in wastewater. Here the focus is on the generation of bio-energy in the form of methane and bio-hydrogen while treating wastewater in an anodic chamber.
The MFC technology can dissolve organic chemicals’ chemical energy directly into electrical energy. On the other side, MEC is capable of generating a product like hydrogen from dissolved organic materials.
The Bioelectrochemical systems employ electrochemically active microorganisms. In the MEC, electrochemically active bacteria oxidize organic matter and generate CO2, electrons and protons. Protons are released into the solution as the bacteria move the electrons to the anode. After that, the electrons join the free protons in solution at the cathode after passing via a wire. Compared to standard water electrolysis (1.23–1.8 V), MECs require a comparatively modest energy input (0.2–0.8 V).
It is possible to produce energy from microalgae through biogas production using anaerobic digestion. Methane can be generated from the digestion of either algal biomass or algae residue. It is a by-product of lipid extraction for biodiesel production.
If the algal lipid content is less than 40%, it has even been proposed that methane synthesis from microalgae without lipid extraction is energetically more favourable than a system where the lipids are extracted before digestion.
Algal digestion usually produces a far lower methane production than is theoretically possible. It has been demonstrated that high-pressure thermal hydrolysis (HPTH) increases methane output during algae digestion by about 80%.
The temperature of the influent wastewater controls the thermal energy. There are various technologies to employ to recover heat. A heat pump is mostly used to harvest thermal energy. The thermal energy available in wastewater would be 41.9 MJ/m3 for every 10 K temperature differential. Another research indicates that heat pumps can supply a net electrical equivalency of 0.26kWh when 1m3 of the effluent is cooled down by 1 K.
Wastewater (grey water), which comes from warm sources including showers, dishwashers, and industrial plants, has a comparatively high temperature when compared to other conventional sources for heat pumps (such as groundwater, geothermal heat, or outdoor air). Additional uses for the wastewater heat include low-temperature sludge drying.
This method of recovery of thermal energy is environmentally friendly and economically competitive. However, it is still underrated. The usage is especially economical, with more than 10,000 PE-sized WWTPs.
The generation of hydro energy through WWTPs is a relatively newer concept or technology. In comparison with chemical energy, recovery of hydro energy in WWTPs is less established. The effluent flow rate and head pressure are two critical elements when developing a hydropower plant. Several factors influence the flow rate of WWTPs on a seasonal basis, and there are significant variances.
For example, a low-head small hydropower plant at the discharge outfall produced very little energy. However, the installation of a hydropower plant in WWTPs may still be appealing because it can be operated all year.
In this article, we have understood a plethora of sustainable solutions for reducing energy consumption in wastewater treatment plants. We also understood briefly about energy recovery methods.
The energy reduction methods will not just save the environment but also reduce the overall running costs. At WiproWater, we help you build and install wastewater treatment plants to improve the overall efficiency of your manufacturing or industrial unit. We aim to reduce freshwater consumption as much as possible and recover all the possible energy and valuable components in wastewater. Let us know your requirements.