ENERGY BENCHMARKING OF WASTEWATER TREATMENT PLANTS SA WATER S EXPERIENCE

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1 ENERGY BENCHMARKING OF WASTEWATER TREATMENT PLANTS SA WATER S EXPERIENCE Michael Corena 1, Daniel Bonini 1, Nirmala Dinesh 1, Rob Macpherson 1 1. SA Water Corporation, Adelaide, SA ABSTRACT Process level energy reviews were completed on 11 SA Water Wastewater Treatment Plants (s) as part of its Energy Efficiency Opportunities (EEOs) Assessment Program. European benchmarks for the key processes were used to measure their energy efficiency and performance against other s. This review resulted in three common themes across all sites which were, high mixer energy consumption, oversizing of equipment and the importance of process selection. Currently SA Water is implementing energy optimisation initiatives at Heathfield which include intermittent mixing, digester control upgrades and advanced aeration control. Early results have been positive with an energy reduction of the total daily site energy consumption by 20% with no negative impacts on process performance. INTRODUCTION The European benchmark values (Baumann & Roth, 2008) have been applied to SA Water s s as a tool for identifying and implementing EEOs. Most recently, process level energy reviews have been undertaken on 11 of SA Water s s including both regional and metropolitan sites with Population Equivalents (PE) ranging from 5,000 up to 1,000,000. SA Water has undertaken three energy benchmarking studies with the most recent being the 11 review. Prior to this, an overall energy benchmarking exercise was undertaken on all SA Water s to identify where the greatest EEOs were (Krampe, 2013). This was followed by a single site process level energy review at Bird in Hand, where the implementation of energy opportunities resulted in 30% savings in electricity consumption and 43% total energy cost savings (Steele et al., 2013). Currently, SA Water is implementing energy efficiency initiatives at Heathfield in response to the findings of the most recent review. The key result of this study was the identification of the amount of energy used by key processes such as the activated sludge process, aeration, sludge thickening & dewatering and digestion. This energy quantification and the application of energy benchmarks has led to a change in how SA Water plans to operate various processes within its s, such as mixing of bioreactors, in the future. In addition to this it will also have a significant impact on the design philosophy applied for both equipment sizing and selection in future upgrades. METHODOLOGY This work embarked on applying energy benchmarking to functional groups within the 11 s. Due to the high costs of installation for energy sub-meters across each process or on individual equipment they are rarely used. As an alternative power monitoring is undertaken via programmable logic controller (PLC) in the SCADA (Supervisory Control and Data Acquisition) system to estimate energy consumption in kwh. Where sub-metering via SCADA was not available or found to be erroneous, a manual calculation method was applied. Based on this methodology, an energy balance was completed for each site and used as a validation technique for this work. The resulting energy data was used to compare against European benchmark values on both a process (or functional group) and equipment level as a measure of efficiency. A comparison was completed against guide and target values which represent the following: Guide Value: The average performance that is achieved by a significant number of European plants with similar capacity and technology. Target Value: The specific energy consumption of the top performers of European plants with comparable technology and size. The guide and target values are presented as a specific energy consumption, which takes into account the energy used per capita per year (kwh/pe/y). The measured specific energy consumption was compared to the guide and target values and a potential energy saving was identified as shown in Figure 1. This potential energy saving is not considered a definitive value, but the comparison was used to identify and prioritise areas for potential improvement opportunities within the s.

Figure 1: Energy Benchmarking Methodology OUTCOMES The process level energy reviews and energy benchmarking of 11 SA Water s resulted in multiple outcomes.

The three common themes realised were, the high energy consumption of mixing processes, oversizing of equipment and the importance of process selection.

2 Figure 1: Energy Benchmarking Methodology OUTCOMES The process level energy reviews and energy benchmarking of 11 SA Water s resulted in multiple outcomes. For the purpose of this paper the common themes across all sites are discussed. The three common themes realised were, the high energy consumption of mixing processes, oversizing of equipment and the importance of process selection. Intermittent Operation of Mixing Processes Mixing processes within SA Water s s were identified as one of the largest consumers of energy. The majority of this energy consumption is a result of the mixing of anoxic zones in activated sludge plants and also in part by digester mixing systems. For Glenelg, the bioreactors and digesters account for 22% and 14% of the total site energy consumption respectively. For the bioreactors, 36% is a result of anoxic mixing (Figure 2) and for the digesters, 74% is a result of the gas mixing system (Figure 3). Figure 3: Glenelg digester energy consumption breakdown The energy benchmarking of mixing operations in both bioreactors and digesters showed that in the majority of cases the SA Water plants exceeded the European guide value. Therefore the implementation of intermittent mixing operation has been raised as a viable solution to reduce the energy consumption. Intermittent operation of bioreactor mixers at two SA Water s has been found to be an effective energy saving measure. This practice has been successfully implemented at Bird in Hand and Christies Beach s which has resulted in energy reductions associated with reactor mixing of 50%. This energy reduction has been achieved without reducing treatment performance and effluent quality. Figure 4 demonstrates that Bird in Hand s effluent quality was not affected by the implementation of intermittent mixing. Therefore SA Water is currently evaluating other sites where this practice is applicable. Figure 4: Bird in Hand effluent total nitrogen Figure 2: Glenelg bioreactor energy consumption breakdown The intermittent operation of digester mixing systems is an option that requires further investigation and trialling before implementation. It is critical that such a practice does not adversely impact the generation of biogas or digester performance. Equipment Sizing Often equipment, such as aeration blowers and UV systems, has been designed to meet the ultimate design loads of the plant and in the majority of cases do not have the necessary turndown ratios required during low flow and loading conditions.

Specific Energy Consumption (kwh/pe/y) This equipment is often the most energy intensive across all treatment sites and oversizing has led to unnecessary energy consumption during these periods.

3 Specific Energy Consumption (kwh/pe/y) This equipment is often the most energy intensive across all treatment sites and oversizing has led to unnecessary energy consumption during these periods. The best example of this is the energy benchmarking of SA Water s aeration processes which shows an exceedance of the average performance of European plants in most cases and top performing plants in all cases (Figure 5). It is acknowledged that this exceedance is in part a result of both conditions and wastewater composition. However, in order to improve energy efficiency better sizing of equipment has been identified as a key consideration. For Port Pirie there are currently two 55 kw positive displacement blowers with VSD (Variable Speed Drives). These blowers have been designed to meet the ultimate design loads but it is expected that during low flow and loading periods they cannot achieve the appropriate turndown ratios. It is expected that this, in addition to other conditions, is playing a role in the poor performance against the benchmark values. This represents a potential to install a third blower which is smaller and has a better turndown ratio for the low flow and loading periods and therefore will improve efficiency. This review has highlighted the importance of considering both ultimate design loads and energy efficiency during the design phase. This example represents how the use of energy benchmarks will influence SA Water s approach to equipment design and sizing in the future. An example of this was in the energy benchmarking of SA Water s sludge thickening processes. At SA Water s larger s Dissolved Air Flotation Thickening (DAFT) technology is used as a means of thickening primary and secondary sludge prior to the digestion process. DAFT uses dissolved air to form tiny air bubbles which float suspended solids to the surface when released into the flotation tank. The suspended solids are then removed from the top of the tank as thickened sludge. DAFT processes within SA Water s are high consumers of energy accounting for 5-10% of the total plant energy consumption as shown in Table 1. This high energy consumption has led to the exceedance of the benchmark value for all SA Water sites which use DAFT technology (Figure 6). Table 1: Proportion of total energy consumption (%) for DAFT plants at SA Water s DAFT % of total plant energy consumption Bolivar 6.7 Bolivar HS 9.5 Christies Beach 4.7 Glenelg 4.1 Heathfield 8.6 Specific Energy Consumption Guide Value Target Value Bolivar Bolivar HS Glenelg Christies Beach Port Pirie Whyalla WRP Heathfield Figure 5: Application of European benchmarks for aeration at SA Water s Process Selection A comparison of current SA Water processes against European benchmark values will be critical in supporting a shift in thinking when it comes to process selection in future upgrades. Following the benchmarking of key processes within the s, it became apparent that achieving the reductions to reach the European guide and target values were unrealistic in some circumstances. This was attributed to the use of more energy efficient equipment options in European s. Figure 6: Application of the European benchmark for mechanical sludge thickening to SA Water DAFT plants Although there are potential improvement opportunities through optimisation it is unrealistic to assume that dramatic reductions in energy consumption can be achieved without compromising the current thickening performance. A majority of the European treatment plants utilise sludge thickening technology such as belt thickeners. These are more energy efficient and also produce higher solids concentrations. SA Water utilises a belt thickener at its Bird in Hand and measuring the specific energy consumption demonstrates the superior energy performance while achieving a thickened sludge concentration of approximately 5%. Figure 7 shows the performance of SA Water s belt thickener against the European benchmark value.

This represents a common theme across process level energy reviews that not all benchmark targets are realistically achievable as a result of the technology being used.

replacement. Intermittent Mixing Heathfield has three activated sludge reactors, each with three mixers.

Figure 8 shows the layout of the mixers in biological reactor number 1 at Heathfield with the same layout followed in the other two reactors.

4 This represents a common theme across process level energy reviews that not all benchmark targets are realistically achievable as a result of the technology being used. Therefore when it comes to process selection in future upgrade works the data on process and energy efficiency will contribute to making design decisions in preference to selecting a like for like replacement. Intermittent Mixing Heathfield has three activated sludge reactors, each with three mixers. Two mixers are situated in each anoxic zone (1 and 2) and one mixer is located in each anaerobic zone (3). Figure 8 shows the layout of the mixers in biological reactor number 1 at Heathfield with the same layout followed in the other two reactors. As a result of the successful implementation of intermittent mixing at other sites, the functionality was implemented at Heathfield to reduce the bioreactor mixer energy consumption. The new functionality installed provides operators with adjustable set-points for intermittent operation of all reactor mixers. The upgrade also allows operators to revert to continuous operation if required. Trials have shown an optimal mixing profile involving an alternate Group 1/Group 2 ON/OFF cycle, where Group 1 is Mixer 1 and Group 2 is Mixer 2 and 3. The ON/OFF time for each group of mixers is a set-point which is a manual input. The mixer operation mode and setpoint screen, as it appears in SCADA, are shown in Figure 9. Figure 7: Application of the European benchmark for mechanical sludge thickening at Bird in Hand Communication to Operations As part of the work completed by Steele et al. (2013) an energy benchmarking tool was developed using Microsoft Excel to provide monthly updates on energy consumption and performance. This tool has been further developed to include the manually calculated energy consumption of process equipment to provide a more complete representation of process performance. The resulting product, a semi-automated user friendly tool, provides operators with energy consumption data that highlights trends and tracks improvements. This tool is envisaged to provide operators regular feedback on their onsite initiatives to optimise energy and thus operating costs. An example of the monthly summary sheet provided to operations is shown in Figure 13 for Whyalla Water Reclamation Plant. Figure 8: Heathfield bioreactor mixers IMPLEMENTATION SA Water is currently in the process of utilising the process energy data and performance comparison against European benchmark values to implement energy efficiency initiatives at Heathfield. These initiatives have been developed through past experiences at Bird in Hand and also through consultation with the operations team. These include intermittent mixing, advanced aeration control (AAC) and digester control upgrades. Figure 9: Heathfield bioreactor mixer intermittent operation screen The initial results, although only one month into operation, have been promising. The key outcome has been the reduction of the run time hours and therefore energy consumption by 50%. The

bioreactor nutrient removal has been maintained (Figure 10) with the implementation of intermittent mixing not altering the effluent quality from its normal trends.

alternates the off mixers. to meet aeration requirements.

The new features implemented were as follows: 1. Re-written control logic for aeration valves 2. Simplified Digester Mode control screen (Figure 12) 3.

5 bioreactor nutrient removal has been maintained (Figure 10) with the implementation of intermittent mixing not altering the effluent quality from its normal trends. The impact of the settling Mixed Liquor Suspended Solids (MLSS) during the mixer OFF periods has also been limited as a result of the short intermittent cycles and optimal mixing profile, which alternates the off mixers. to meet aeration requirements. As a result of these issues an upgrade on the digester control system was completed to improve the operation performance and achieve greater efficiency. The new features implemented were as follows: 1. Re-written control logic for aeration valves 2. Simplified Digester Mode control screen (Figure 12) 3. Ability to select Aeration only or Intermittent Aeration 4. Simple set-points for aeration ON and OFF time 5. Operator adjustable set-points for minimum and maximum valve position 6. Operator adjustable Operate Mixers in Aeration OFF time 7. Operator adjustable PID loop tuning parameters Figure 10: Heathfield nutrient data Digester Control The aerobic digester at Heathfield was identified as a key target for improvement. The process level energy review revealed that it accounts for 26% of the total site energy consumption (including aeration energy) as shown in Figure 11. In addition to this, consultation with the operations team revealed that there were a number of fundamental control issues with the aerobic digester system which were causing inefficient and sub-optimal operation. Figure 12: Heathfield simplified Digester Mode control screen This work has already resulted in a noticeable drop in energy use, primarily due to the fact that operators are now confident to let the digesters operate in automatic DO control. Previously digester 1 would have the air control valves in manual and open 100%, resulting in a large waste of energy particularly at night and on weekends when there are no septic loads. Figure 11: Heathfield energy consumption breakdown The most critical issue for the digesters was that there were maximum open percentages for the control valves programed into the system which were not operator adjustable. This resulted in limited air being delivered to the digesters, contributing to the DO/ORP set-points not being achieved. The operators were therefore forced to run valves in manual, at 100% open, continuously Advanced Aeration Control Aeration for biological reactors is one of the largest consumers of energy within all s. Therefore AAC is being evaluated as an initiative to reduce the energy requirement of aeration blowers. Most Open Valve (MOV) logic, an AAC strategy, is currently in the process of being implemented and evaluated at Heathfield. MOV logic is a control strategy for operating aerated bioreactor systems. The principle behind MOV control can be represented by equation 1. P B = P PF + P CV + P D + P H Eq. 1 (Kapocius, 2012)

6 Kapocius (2012) defines this equation as the pressure that must be provided by the blower (P B ) as equal to the sum of the pressure lost across piping and fittings (P PF ), throttling air control valves (P CV ), diffusers (P D ) and the aerated water head that the air has to push through (P H ). MOV control is designed to operate the aeration system such that at least one of the air control valves is close to being fully open and therefore P CV is minimised. Practically, this is achieved by allowing the air manifold pressure to become dynamic. If the most open valve begins to throttle, then the control system will lower the pressure set-point so the valve ceases throttling and friction loss is reduced. On the other hand if the reactor zone DO set-point associated with the MOV cannot be maintained at the maximum opening position, then the system will increase the pressure set-point until the DO setpoint is achieved. When operating properly, MOV can typically result in average discharge pressures of 3-7 kpa lower than the conventional constant pressure control setting (Jenkins, 2013). While this number may appear to be small, fan affinity laws show that these small reductions in discharge pressure translate to larger energy savings of around 3% per kpa. Sydney Water has reported average savings of 10% for plants that have converted from constant pressure control to MOV pressure control (Kapocius, 2012). Currently, the aeration system at Heathfield is operated in constant manifold pressure mode. The blowers maintain the local pressure set-point in the main aeration header which is set by the operator. Once implemented, the Heathfield aeration system will operate on MOV control logic and it is expected that after optimising the process, energy savings will be realised. Plant Performance Energy Consumption The implementation of the digester control upgrade and intermittent mixing operation has resulted in an immediate impact on the site energy consumption. Prior to the energy efficiency initiatives being implemented the total daily site energy consumption at Heathfield was on average 2550 kwh (Figure 14). After the digester control upgrade in early December 2014 this value was reduced to an average of 2220 kwh/d. In mid- December 2014, the implementation of intermittent mixing resulted in a further reduction to an average of 1980 kwh/d. Therefore the implementation of two energy efficiency initiatives has resulted in an average saving of 550 kwh/d. This is equivalent to a reduction of the total daily site energy consumption by 20%. In addition to this it is expected that once advanced aeration control has been implemented and optimised further savings will be achieved. CONCLUSION The major outcome of this work has been the quantification of energy consumption by key processes within SA Water s and the successful application of benchmarking principles to identify improvement opportunities. The organisation is now more focused on identifying where excess energy is being consumed in order to maximise the performance of existing infrastructure through process optimisation. This is demonstrated through the implementation of energy efficiency initiatives at Heathfield which have resulted in a reduction of 20% of the total daily site energy consumption. It is expected that the implementation of successful initiatives at Heathfield, such as intermittent mixing, will result in the application at other sites where applicable. It has also resulted in a shift in thinking about the importance of process selection and equipment sizing in achieving energy & process efficiencies at SA Water s. It is anticipated that the findings of this work will contribute, in the future, to making decisions in process upgrades and design instead of opting for like for like replacements. This paper continues to emphasise the benefits of applying energy benchmarking as a tool for identifying improvement opportunities and potential cost savings. ACKNOWLEDGMENTS The author would like to acknowledge Joerg Krampe and Rowan Steele for their contributions and work in energy benchmarking at SA Water. The author would like to acknowledge the Heathfield operations team for assisting with and monitoring the performance of the implemented energy efficiency initiatives. REFERENCES Baumann P. & Roth M. (2008). Senkung des Stromverbrauchs auf Klaranlagen (Reducing Power Consumption in Wastewater Treatment Plants). Leitfaden fur das Betriebspersonal, Issue 4. Jenkins, T. E., Aeration Control System Design : A Practical Guide to Energy and Process Optimization. USA: John Wiley & Sons. Kapocius, A. (2012). Aeration Cost Savings Using Dynamic Pressure Setpoint Control. National Operations Conference Darwin, Sydney Water. Krampe J. (2013). Energy benchmarking of South Australian s. Water Science & Technology: Steele R., Krampe J. and Dinesh N. (2013). Process Level Energy Benchmarking as a Tool to improve the Energy Efficiency of Wastewater Treatment Plants. AWA journal

7 Figure 13: Whyalla Water Reclamation Plant energy benchmarking tool summary page provided to operations

8 Prior to energy efficiency initiatives Post digester control upgrades Post intermittent mixing implementation Figure 14: Heathfield total daily site energy consumption

Best practice in monitoring process, operation and maintenance of wastewater treatment plants 12 September 2011, Bucharest

Best practice in monitoring process, operation and maintenance of wastewater treatment plants 12 September 2011, Bucharest Steve Russell WRc PLC Swindon UK Steve.russell@wrcplc.co.uk WRc plc 2011 Plan