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Yasmin, R.; Amin, B.M.R.; Shah, R.; Barton, A. Demand Response in Wastewater Treatment Plant. Encyclopedia. Available online: https://encyclopedia.pub/entry/54435 (accessed on 17 May 2024).
Yasmin R, Amin BMR, Shah R, Barton A. Demand Response in Wastewater Treatment Plant. Encyclopedia. Available at: https://encyclopedia.pub/entry/54435. Accessed May 17, 2024.
Yasmin, Roksana, B. M. Ruhul Amin, Rakibuzzaman Shah, Andrew Barton. "Demand Response in Wastewater Treatment Plant" Encyclopedia, https://encyclopedia.pub/entry/54435 (accessed May 17, 2024).
Yasmin, R., Amin, B.M.R., Shah, R., & Barton, A. (2024, January 27). Demand Response in Wastewater Treatment Plant. In Encyclopedia. https://encyclopedia.pub/entry/54435
Yasmin, Roksana, et al. "Demand Response in Wastewater Treatment Plant." Encyclopedia. Web. 27 January, 2024.
Demand Response in Wastewater Treatment Plant
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This entry is adapted from the peer-reviewed paper 10.3390/su16020731

The transition from traditional fuel-dependent energy systems to renewable energy-based systems has been extensively embraced worldwide. Demand-side flexibility is essential to support the power grid with carbon-free generation (e.g., solar, wind.) in an intermittent nature. As extensive energy consumers, commercial and industrial (C&I) consumers can play a key role by extending their flexibility and participating in demand response. Onsite renewable generation by consumers can reduce the consumption from the grid, while energy storage systems (ESSs) can support variable generation and shift demand by storing energy for later use. Both technologies can increase the flexibility and benefit by integrating with the demand response. 

commercial and industrial consumers demand response distributed energy resources

1. Introduction

Industry-wise load and operational characteristics can differ from each other and require distinctly different analyses to identify individual DR potential. Many of the past studies reviewed the application of DR in cement [1][2][3][4], steel [1][2][3], aluminium [2][3][4], or the food industry [1][2]. The water industry is one of the extensive energy users; however, it lacks DR-based research attention. Australia’s water industry accounts for about 19% of its energy consumption [5], whereas treating wastewater requires significant energy use [6].
WWTPs consume about 1% of the total energy use of a country. They are also a significant greenhouse gas emitters [7]. The stringent quality requirements for treating wastewater need intensive energy, where electricity costs can vary from 25% to 50% of the total cost depending on the treatment type or plant size [8]. However, some energy-consuming processes, such as pumping or aeration, offer flexible operation, enabling the plants to provide DR services [9]. Additionally, usually the peak load operation of these plants coincides with the peak period of grid demand, which makes WWTPs a potential DR participant [10]. A WWTP can also be considered a chemical and thermal energy source, due to its energy generation capacity [11]. Energy management of such plants is pragmatic and beneficial for the plant in terms of cost reduction. The WWTP operation is typically executed by pumps, air compressors, dewatering machines, surface aerators, analysis equipment, mixers, and other machinery and equipment. There are four major steps in the treatment process named as the preliminary, primary, secondary, and tertiary (advanced) process. In the preliminary process, wastewater is pumped into the system through screening to remove large particles such as paper and rocks, before going on to primary treatment [12]. Primary treatment involves the removal of suspended particles and organic matter by sedimentation, which produces sludge [12]. Aeration and secondary clarification, typically involving biological treatment such as the activated sludge process, are the main parts of the secondary treatment [12][13]. The sludge produced from the primary and secondary treatments goes for anaerobic digestion and produces biosolids after dewatering for agricultural/commercial use. Biogas can also be captured and used to generate electricity using CHP units [12][13]. The effluent from the secondary treatment stage finally goes for tertiary treatment. Using chemical disinfection or UV filtration, the water is made ready for reuse or discharge to the surface water [12][13].
Although the WWTP operational steps require substantial energy use, some of the processes provide flexible operation and DR potential to WWTP through load-shifting strategies [14]. The flexible resources and DR potential within a WWTP are reviewed and included in the sub-sections next.

2. Flexible Resources

2.1. Aeration Process

The aeration process promotes biochemical reactions [15], where the air is introduced into the bioreactors at the secondary treatment [14]. This continuous process facilities the decomposition of organic matter to an acceptable level through biological degradation, accounting for about 45–60% of the total energy used for treatment [14]. To meet the wastewater quality requirements with energy saving, an analysis of aeration control is critical [15]. Among the three components of the aeration system, including airflow generation by blowers, airflow distribution, and aeration tanks, aerator blowers consume most of the energy [8][12]. Switching off the aeration process for justified time limits can provide DR potential while considering appropriate control parameters [16][17]. Regulating the dissolved oxygen (DO) concentration is a common process for aeration control [18]. Over-oxygenating stored wastewater during off-peak time can provide load-shifting opportunities by delaying the aeration process to avoid energy consumption during peak periods [14]. Nevertheless, the DO concentration can vary according to diurnal and seasonal variations. Hence, to avoid the risk of degraded effluent quality, maintenance of strict control parameters with monitoring of oxygenation levels should be highly considered [14]. However, further research is required to investigate DR potential using over-oxygenation modelling [14].

2.2. Pumping

Pumping is an essential process in WWTP encompassing raw water pumping, in-plant, and finished water pumping [19]. The use of pumping throughout the treatment process makes it the second highest energy-consuming process, taking up about 15–70% of total energy use [14]. Subject to sufficient storage, pumping can be used in load shedding and shifting strategies. The use of lift pumps with low ramp rates in intermittent operation or turning off the return pump for a period in an activated sludge application, was found to have the potential for load shifting. However, further research is required regarding intermittent operations, and interconnected and interdependent system effects [14].

2.3. Built-in Redundancy

Total WWTP design capacity is usually notably greater than the used operational capacity, which can offer excess storage capacity. As discussed in [14], this additional capacity can be used to store wastewater at different stages of treatment—in untreated form or partly/fully treated form to avoid high-price periods. At off-peak time, further processing or final discharge of this stored water can save energy costs. Further, the peaks of wastewater’s daily inflow patterns match the peak electricity demand, offering huge potential for energy savings through load shifting while providing pumping flexibility [14]. However, these redundancies aim to counteract the risk of heavy rainfall events. Hence, utilising the opportunity for DR requires a prudent appraisal of high-quality weather forecasts to avoid any risk of over-stressed systems and discharging untreated wastewater including operational contingencies [14].

2.4. Onsite Generation

The anaerobic digestion during sludge processing results in biogas production which is a valuable source of energy for the flexibility. This biogas, including natural gas (methane), can generate electricity using CHP units, and shifting CHP operations to generate energy at different periods allows for curtailing grid energy usage [13][16]. Australia is effectively using this technology to produce energy by biogas, where Sydney Water WWTP meets about 21% of the operational energy requirements by using this technology [8]. Furthermore, WWTPs usually have large areas that can allow them to install PV panels and batteries. These DERs can assist in using electricity at a low price and support DR events [6]. Besides biogas, energy recovery through hydrogen production is another potential technology [20]. Hydrogen can be produced from biogas through the thermochemical method, usually by steam reforming or the partial oxidation process in the presence of a suitable catalyst. A membrane reactor is another technology that uses a membrane to separate the hydrogen from the non-hydrogen particles of the biogas feed [21]. Further detail on each method can be found in [21]. Green hydrogen (solar to hydrogen) production using proton exchange membrane (PEM) technology was also studied in [22]. The use of treated effluent water for the electrolysis process provided an efficient water reclamation strategy with increased revenue. In [22], the authors showed that applying PEM electrolysis technology provided oxygen as the only by-product without any carbon emissions. PEM technology can effectively produce pure hydrogen using renewables [23]. Highly purified input water is essential for efficient hydrogen production, so the reverse osmosis (RO) process is used for improved effluent quality [22]. Although the system requires high capital investment, the revenue earned by hydrogen selling could cover the investment cost shortly. Microbial fuel cells (MFC) are emerging technologies that can simultaneously treat wastewater and produce bioelectricity to harvest more renewable generation in a sustainable way. In this way, hydrogen can be obtained as a by-product [24][25]. MFC uses the inherent chemical energy of organic or inorganic matter to produce electricity through electrochemical reactions [24]. Although it shows potential advantages such as reduction in waste aeration and solid sludge production and odour control; however, the technology is yet to be matured for the commercial use [26]. Rescheduling the processes, including backwash pumps, biosolids thickening, and dewatering operation to off-peak periods [12] or diverting flow equalization basin [13] are the other measures to reduce peak demand in WWTPs.

3. DR Potential in WWTP with Increased Flexibility

DR potential of WWTP through load shifting or ancillary services has been discussed in past studies [18][27]. However, the strict quality restrictions pose some concerns to implement the flexibility in the WWTP. As discussed in Section 6.1, aeration control is a flexible resource where biological nitrogen removal can provide flexible up–down regulation in the electricity market. However, the permissible limit of ammonium and total nitrogen (TN) should be considered [7]. Intermittent operation of aeration by switching off the blower can affect the DO concentration and result in effluent turbidity. It might lead to a stronger aeration activity requiring increased energy use and might offset DR benefits [10], and hence requires well-justified means to ensure flexibility.
In [28], the authors analysed the capability of different residential and industrial loads to provide frequency regulation services. The experiment showed that the control of the non-critical loads in WWTP, such as induction motors (used for pumping water and moving cleaning brushes), enables provision of ancillary services as frequency regulation. Based on the signal sent by the frequency controller, the plant control system was reprogrammed to interrupt the processes prioritising the system constraints [28].
The application of CAES technology in aeration system was examined for demand shifting [29]. Using an air compressor, air can be stored in the tank during off-peak periods and used for aeration during high-price periods. In addition, various factors are required to be considered, including wastewater characteristics and biological energy demand. Also, the operation of the activated sludge process needs to be accounted for [29]. However, CAES poses particular geographical demand and is not a well-accepted technology in Australia [30].
The VPP-based ancillary services can be obtained by aggregating the WWTPs. However, to provide a positive control reserve, flexible plant components should be examined to identify the maximum downtime, ensuring the smooth operation of WWTP processes [31].
Having substantial DR potential for WWTP, the concern of quality degradation is a significant barrier to DR implementation and obtaining the benefits of cost saving. Focusing on this concern, the study in [13] investigated the load-shifting effects on a full-scale WWTP in California. The three load-shifting strategies reported in [32] are—CHP generation time shifting, diverting flow-equalisation basin, and discharging an onsite battery. It reported that the assets which do not affect wastewater quality treatment as the CHP unit can be used for load shifting. However, the additional infrastructure capable of load stabilising like batteries, is crucial in meeting the energy reduction commitment. Coupling biogas with solar energy can also reduce energy demand and increase flexibility in plants’ energy management strategies [33]. In California, a 7 MW/34 MWh network of battery arrays is installed at six water treatment, recycling, and pumping facilities. The stored energy is purchased during cheap price periods allowed to support DR events [6].
A more sustainable approach to shifting the energy demand of WWTP is studied by integrating an electrolysis process with an activated sludge process in [27]. It also considered the shifting energy demand during the aeration process using a compressed oxygen energy storage (COES) unit. Stored oxygen, the by-product of the electrolysis process, provided longer durations in load shifting than intermittent aeration without interrupting the process and affecting wastewater treatment quality. Eliminating the use of the aeration blower during peak periods could reduce energy bills. The hydrogen produced can be stored to produce electricity using a fuel cell or sold to the market to add revenues. Moreover, WWTP is also a source of thermal energy; hence, potential research should be performed focusing on extracting thermal energy from the WWTP system to meet thermal demand. Heat generated by a CHP unit using the biogas produced from the digester and the heat reclaimed from the wastewater effluent using a heat pump met internal heating demand. Further, the excess thermal energy was supplied to a residential site resulting in revenue earning [34].
Dynamic load shifting in a hybrid WWTP comprised of centralised and satellite water resource recovery facilities (WRRF) was studied to curb energy cost, power demand, and greenhouse gas emissions (GHG) [35]. The study [35] performed three scenarios. In the first scenario, the ToU DR program was used for load shifting by diverting the influent to the main plant, while regulating the pre-set influent control parameter. In the second scenario, equalisation of the flow of the centralized plant performed, which improved the process stability and the variance in operation. The indirect GHG emissions from the use of energy for treating wastewater was reduced in the third scenario by shifting the load to the periods of lower emission intensity. The interconnection between the facilities provided the benefits of load diversion without shutting down any process. The result gives a decrease in power demand up to 25%, use of energy 4%, operating cost 8.5%, and indirect GHG emission 4.5% [35].

4. Outlook

The energy demand of a WWTP is influenced by the size, location, treatment process, age, wastewater quality requirements, and others [36]. Understanding tariff structure is crucial to assume a plant’s energy cost. Energy consumption can also vary based on aeration control strategies using different control parameters (TN, DO, etc.) [37]. So, a cost analysis considering the tariff structure and different charges in conjunction with an efficient control strategy can reduce energy costs. Capacity charges of such plants are usually estimated against the peak demand over 12 months. A notable reduction in peak demand can result in reduced capacity charges [8]. However, reluctance and concern in participating in DR by the operators due to the absence of informed education about the program and energy data relevancy show the importance of educating the concerned operators and relevant staff, which can increase WWTP DR participation [13]. WWTP is a source of valuable energy, where renewable energy can be extracted from the biogas generated from the process, or the effluent, even the influent through advanced technology, which requires more research attention and support to utilise these opportunities in a sustainable way.

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Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Roksana Yasmin
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B. M. Ruhul Amin
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Rakibuzzaman Shah
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Andrew Barton
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Update Date: 29 Jan 2024
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