Level 2 Energy Audit. NRSBU Bell Island Wastewater Treatment Plant. Engineering Advisor Nelson Regional Sewerage Business Unit (NRSBU)

Transcription

1 Level 2 Energy Audit NRSBU Bell Island Wastewater Treatment Plant Performed for: Johan Thiart Engineering Advisor Nelson Regional Sewerage Business Unit (NRSBU) June 212

2 CONTENTS EXECUTIVE SUMMARY INTRODUCTION BACKGROUND PURPOSE OF ENERGY AUDIT ENERGY REVIEW PERIOD HISTORIC ENERGY USE HISTORIC ELECTRICITY USE ELECTRICITY USE CONSUMPTION PROFILES ELECTRICITY SPLIT TARIFF REVIEW ELECTRICITY PROCESS DESCRIPTION PRELIMINARY TREATMENT PRIMARY TREATMENT SECONDARY TREATMENT SOLIDS TREATMENT AUXILIARY PROCESSES AND EQUIPMENT ENERGY MANAGEMENT OPPORTUNITIES ELECTRICITY SUPPLY MAIN PROCESSES HEATING, VENTILATION AND AIR CONDITIONING (HVAC) LIGHTING MISCELLANEOUS EQUIPMENT APPENDIX A SITE MAP WITH MAIN PROCESS LOCATIONS APPENDIX B TYPICAL ELECTRICITY BILL APPENDIX C ELECTRICITY METERS APPENDIX D POWER FACTOR READINGS APPENDIX E QUOTES APPENDIX F TECHNICAL RESOURCES NRSBU BISTP Level 2 Energy Audit

3 EXECUTIVE SUMMARY An energy audit at the Nelson Regional Sewerage Business Unit (NRSBU) Bell Island Sewerage Treatment Plant (BISTP) was conducted by Enercon for the following reasons: To find methods to improve energy efficiency; o An energy audit had never been performed on the site since its inception in 1982 To develop an energy balance of the entire plant; To analyse the optimal performance of the overall waste water treatment process on site. Overall, the plant is operating very energy efficiently, with many energy conserving features and practices already in place. Site staff have a proactive and knowledgeable approach towards energy management, which is encouraging when considering long term efficiency. The Bell Island Sewerage Treatment Plant is administered by NRSBU, and is operated by a contractor, CPG. Reduce Energy Consumption The Bell Island Sewerage Treatment Plant has an annual energy spend of approximately $535, based on the year ending March 212. The site was found to be highly energy efficient in regards to the operation of existing processes, and would fit in the top 1% of Energy audits we have done for operating efficiency. Examples of this efficiency are: Variable speed drives are installed as and where appropriate; Site equipment is regularly maintained and reviewed, including the cost and payback period of more efficient approaches; Power factor is universally high, ensuring no supplied power is wasted through poor distribution; Operational issues are quickly dealt with, resulting in little equipment left operating to compensate for the issues; Process equipment is easily controlled and monitored from the SCADA system in the main office; Staff are knowledgeable about what each water treatment process is trying to achieve and what needs to be operating as a result; Air conditioning use is kept to a minimum; Lighting is sparingly used and relates closely to occupancy (e.g. there were almost no lights on overnight when staff had left the island). NRSBU BISTP Level 2 Energy Audit 1

4 Electricity is not currently generated on site. A number of Energy Management Opportunities (EMOs) have been identified that, when implemented, will decrease the electricity consumed (and therefore the cost). Table.2 summarises the energy consumption and the savings that can be made. Note that average energy prices from February 211 to January 212 have been used. Table.2: Summary of Annual Energy Usage for Bell Island Sewerage Treatment Plant Electricity (kwh e ) Electricity Generated (kwh e ) Electricity Imported (kwh e ) Total Cost ($) Cost Savings ($) Cost Savings (%) Current 3,824,1 3,824,1 $534,9 - - If T1 met 3,682,1 3,682,1 $512,4 $22,5 4.2% If T1 & T2 met 3,68,3 3,68,3 $512,2 $22,7 4.2% Table.3 provides benchmark figures for the energy consumption of the plant. The total electricity consumption can be expressed with index values of.71kwh e /m³ of wastewater and 9.8 /m³ of wastewater, or 9,161 kwh e /tonne ($1,281/tonne) of biochemical oxygen demand (BOD) removal. Table.3: Summary of Benchmark Energy Consumption Figures Annual Wastewater Flow Annual BOD (m³) tonne (kwh e /m³ Wastewater) Consumption Benchmark (kwh e /tonne BOD) Cost Benchmark ( /m³ Wastewater) ($/tonne BOD) Current 5,456, , ,281 If T1 met 5,456, , ,228 If T1 &T2 met 5,456, , ,227 Table.4 compares the energy consumption of the Bell Island Sewerage Treatment Plant to two other wastewater treatment plants in New Zealand (North Shore and Christchurch). Table.4: Summary of Benchmark Energy Consumption Figures BISTP North Shore Christchurch (kwh e /m³) ( /m³) (kwh e /m³) ( /m³) (kwh e /m³) ( /m³) Current As can be seen in Table.4, the Bell Island Sewerage Treatment Plant is more energy intensive than the North Shore and Christchurch wastewater treatment plants. North Shore and Christchurch wastewater treatment plants both use anaerobic digesters, while the Bell Island Sewerage Treatment Plant uses aerobic digesters. The Bell Island Sewerage Treatment Plant has the potential to be a net exporter of energy (as is the case at the Christchurch wastewater treatment plant), were it to convert from aerobic to anaerobic digesters. This measure is discussed further in EMO 2.1 of this report, which states 4,4, kwh/year could potentially be generated / NRSBU BISTP Level 2 Energy Audit 2

5 saved using anaerobic digesters, compared to the 3,824,1 kwh/year used over the 12 month historical period analysed. Significant energy saving opportunities with short paybacks can be found by: Having CPG pay for energy accounts as part of their performance contract Replacing the flow meter for gravity fed outflow Removing the time-of-use metering at the dewatering plant Possible longer term investments include: Anaerobic Digesters Gravity Belt Thickener (GBT) use instead of Dissolved Air Floatation unit (DAF) use This audit needs to be seen as the first step to reducing and managing energy costs. The next phase is to implement the recommendations from this report. Thereafter, ongoing energy management will be needed to ensure energy savings are realised into the future. Energy management will also ensure operating and maintenance issues are identified early and assist in improving the energy awareness of employees. Energy audits should be conducted at sites every 3 5 years to achieve good energy management. Continual energy management will ensure operating and maintenance issues are identified early, new technologies are utilised as they become available and cost efficient, and assist in improving the energy awareness of staff. A summary of the Energy Management Opportunities (EMOs) recommended is shown in Tables.5,.6 and.7. Note that the capital costs are budget figures only. A full description of each of these EMOs is given in Section 5. EMO Designation: The decimal number labels each EMO in a sequence. The first number is coded: 1 Electricity Supply 2 Main Processes 3 Lighting 4 HVAC 5 Office Equipment Note that some EMOs are further separated into the following: a b c Paybacks less than 1 year Paybacks between 1 and 5 years Paybacks over 5 years NRSBU BISTP Level 2 Energy Audit 3

6 Table.5: T1 EMOs (payback less than one year) EMO Action Capital Cost Annual savings Simple Payback Include energy account payments in the performance contract for CPG Remove the non-half-hourly metering at the dewatering plant Negotiable $~21, N/A $5 $2,7.2 years 2.2 Replace the flow meter for gravity fed outfall $6, $19,5.3 years 2.4 Optimise O 2 levels in Aeration Basin Management Needs further investigation Needs further investigation Place the Aeration Basin Distribution-Board Room fan on a thermostat Use one freezer instead of two in the room next to the tea room $15 $25.6 years $ $8 Immediate Total T1 (Note: exc. EMO 1.1) $6,7+ $22,5+.3 years Table.6: T2 EMOs (payback between one and five years) EMO Action Capital Cost Annual savings Simple Payback 4.1 Switch outside lighting off during the day $29 $ years 4.2 Occupancy sense freezer room lights $15 $7 2.2 years Total T2 $44 $ years Total T1 + T2 $7,1+ $22,7+.3 years Table.7: T3 EMOs (payback over five years) EMO Action Capital Cost Annual savings Simple Payback 1.3 Investigate metering requirements for the ponds Needs Further Investigation $1,75 - $3,5 Needs Further Investigation 2.1 Use anaerobic digesters instead of aerobic digesters $7,2, $4, 18 years 2.3 Investigate using the Gravity Belt Thickener instead of the Dissolved Air Floatation unit for secondary sludge thickening Needs Further Investigation $~$7, Needs Further Investigation Note: All costs in this report exclude GST. NRSBU BISTP Level 2 Energy Audit 4

7 1 INTRODUCTION 1.1 Background Occupying 53ha of Bell Island on the Waimea Inlet, the Bell Island Sewerage Treatment Plant (BISTP) serves the communities of Nelson South, Richmond and Mapua. The plant was commissioned in 1983, with an original design population of 33, people, and the plant consisted of a fully mixed aeration basin, three facultative oxidation ponds (in parallel), two maturation ponds (in series) and a tidal discharge. As of 27, the plant had a design population of 133, people. Bell Island Sewerage Treatment Plant processes approximately 5.5 million cubic meters of wastewater per year. The plant provides primary and secondary treatment. Two-thirds of this flow originates from residential sources; the remaining one-third comes from commercial and industrial producers. However, the industrial users produce two-thirds of the biochemical oxygen demand (BOD). Peak flows and loads are highly variable due to the combined effects of storm water filtration and the seasonal nature of industrial food processing activities. Treated effluent is discharged into the inlet on the outgoing tide. Stabilised sludge (bio-solids) is beneficially applied to forests on Rabbit Island. The treatment plant consists of an aeration basin, clarifier, Dissolved Air Flotation unit (DAF) and an Autothermal Thermophilic Aerobic Digestion (ATAD) plant that treats captured solids to produce bio-solids. A system of pumps and pipework transfers bio-solids to Rabbit Island. The oxidation pond system at Bell Island Sewerage Treatment Plant consists of three facultative ponds in parallel and two maturation ponds in series. Effluent from the last maturation pond is discharged into the Waimea Inlet via an outfall pipeline and diffusers. To manage increasing flows and tightening environmental requirements, a number of process upgrades have been made since the plant s construction. Following a review in 1992 the following were installed: Desludging of the oxidation ponds was undertaken from due to the overloading (and subsequent malodour generation) of these ponds caused by stratification and organic load build up in excess of treatment capacity. Mechanical aeration mixing (completed 1996) Clarifier and Autothermal Thermophilic Aerobic Digestion (completed 1996). This phase of the project was implemented to reduce the loadings entering the oxidation ponds, thus reducing odour nuisance from the plant. From 23-25, a retrofit of the plant took place that included installation of the Dissolved Air Flotation unit. More recently, a Gravity Belt Thickener (GBT) has been installed on site. This was used for 2 months, before it was decided the system was not required at that stage. As of 27, the following developments were being considered in future planning. Anaerobic digestion is explored in this report. NRSBU BISTP Level 2 Energy Audit 5

8 27/8 Inlet load reduction (primary clarifier) 28/9 Outfall capacity upgrade 21/11 Expand bio-solids treatment facilities 213/15 Anaerobic Digestion and Co-generation 215/16 Nitrogen / phosphorous Removal 215/16 Pond De-sludging A 15 year review of all site equipment is typical, although exceptions could be made for systems that are proven to be more energy efficient or more productive. A full process description can be found in Section 4. Bell Island Sewerage Treatment Plant has the following operating hours: Operation Days/week Hours/day Comments Process Control Room BMS remotely accessible Offices Mon Fri employees 1.2 Purpose of Energy Audit The Nelson Regional Sewerage Business Unit decided to have an energy audit performed at the Bell Island Sewerage Treatment Plant for the following reasons: To find methods to improve energy efficiency To develop an energy balance of the entire plant To analyse the optimal performance of the overall waste water treatment process on site. 1.3 Energy Review Period The energy review was performed with two day-time site visits. An evening inspection was performed as well. Energy figures throughout the report are based on energy data from February 211 to January 212 (unless otherwise specified). NRSBU BISTP Level 2 Energy Audit 6

9 2 HISTORIC ENERGY USE 2.1 Historic Electricity Use Table 2.1 shows a summary of the net electricity consumption and cost for the year ending March 212 for the entire Bell Island Wastewater Treatment Plant. The Net Energy Cost represents the total marginal charge for each kwh whereas the total electricity cost includes fixed costs such as line charges. Please note: Historical cost figures are based on Meridian charges prior to the switch to Trustpower in February 212. Energy savings calculations use projected Trustpower TOU rates from June 212 to May 213. Table 2.1: Electricity Consumption (April 211 March 212) Electricity Consumption (kwh e ) Net Energy Cost TOTAL Electricity Cost 3,824,1 $482,3 $534,9 Table 2.2a shows a summary of the electricity consumption for meter 48125NTFE5 (Aeration Basin, Dissolved Air Floatation Unit, Return Activated Sludge, Waste Activated Sludge) for the year ending March 212. This meter accounts for 63% of the entire plant energy use. Table 2.2a: Electricity Consumption for Meter 48125NTFE5 (Feb 211 Jan 212) Electricity Consumption (kwh e ) Net Energy Cost TOTAL Electricity Cost 2,419,8 $295,4 $334,3 Table 2.2b shows a summary of the electricity consumption for meter 597NT746 (Digesters, Secondary Clarifier, Workshop, Sludge Tanks / Pumps). This meter accounts for 32% of the entire plant energy use. Table 2.2b: Electricity Consumption for Meter 597NT746 (Feb 211 Jan 212) Electricity Consumption (kwh e ) Net Energy Cost TOTAL Electricity Cost 1,238,9 $154,8 $166,8 Table 2.2c shows a summary of the electricity consumption for meter 32543NT962 (Inlet, Screens, Outfall, Wash-water etc.). This meter accounts for 3% of the entire plant energy use. Note: Net energy cost is higher than total electricity cost for this meter as large fixed cost refunds occurred during the year analysed. Table 2.2c: Electricity Consumption for meter 32543NT962 (Feb 211 Jan 212) Electricity Consumption (kwh e ) Net Energy Cost TOTAL Electricity Cost 12,8 $22, $21,2 Table 2.2d shows a summary of the electricity consumption for meter (Pond Aerators, Farmers Water). This meter was not analysed in detail as it covers a large area with comparatively small energy use (1% of total BISTP use). NRSBU BISTP Level 2 Energy Audit 7

10 Table 2.2d: Electricity Consumption for meter (Feb 211 Jan 212) Electricity Consumption (kwh e ) Net Energy Cost TOTAL Electricity Cost 42,6 $9,2 $9,6 The Bell Island Wastewater Treatment Plant also has a small meter (592NT-B86) for the Dewatering Plant and Electrician s Workshop, but as Table 2.2e shows, this only consumes around 1,8 kwh/year. The meter costs Bell Island Wastewater Treatment Plant $2,9, of which only $4 is for energy that only keeps this filter on standby. It is recommended this meter be removed. Table 2.2e: Electricity Consumption for meter (592NT-B86) (Feb 211 Jan 212) Electricity Consumption (kwh e ) Net Energy Cost TOTAL Electricity Cost 1,8 $4 $2,9 2.2 Electricity Use Consumption Profiles Figure 2.1 compares the monthly electricity consumption for the main site to the amount of wastewater treated for January 211 March 212. The amount of electricity consumption has a very weak relationship to the throughput of wastewater with much of the equipment operating at a set load throughout the year. As expected, the electricity consumption profile is relatively flat throughout the year. However, the discharge rate varies by as much as 4% (e.g. between February and June 211) due to changes in weather conditions, while the corresponding electricity use variation is only around 3%. Note that the inflows become high with heavy rains due to storm water entering the waste water catchment area. The energy use per m 3 of water also varies greatly. The energy use per unit of discharged treated water was particularly high for February April , Electricity use and water discharge 1.8 Energy use / m3 of water 7, 1.6 Monthly m3 / Monthly kwh 6, 5, 4, 3, 2, kwh/ m ,.2. Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11 Oct-11 Nov-11 Dec-11 Jan-12 Feb-12 Mar-12 Average Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11 Oct-11 Nov-11 Dec-11 Jan-12 Feb-12 Mar-12 Average Discharge (m3) Electricity Use (kwh) Discharge Inflow Figure Electricity Consumption and Wastewater Flow Figure 2.2 reinforces the findings of Figure 2.1. The electricity consumption and generation have relatively flat profiles and do not vary significantly with wastewater flow. Only 26% of the variation in electricity consumption can be NRSBU BISTP Level 2 Energy Audit 8

11 attributed to changes in treated water discharge (as indicated by an R² value of 26%). Only 9% of the variation in electricity consumption is related to changes in wastewater inflow. Discharge (m3) vs. Energy use (kwh) Inflow (m3) vs. Energy use (kwh) 4, 35, y =.672x R² = , 35, y =.4x R² =.861 3, 3, 25, 25, kwh 2, 2, 15, 15, 1, 1, 5, 5, 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, kwh 8, 9, Water discharge (m3) Water discharge (m3) Figure Electricity Consumption vs. Wastewater Flow Biochemical oxygen demand (BOD) has only been logged at Bell Island Wastewater Treatment Plant since September 211. BOD is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. BOD is sometimes used as a gauge of the effectiveness of wastewater treatment plants. For Bell Island Sewerage Treatment Plant, 2/3 of BOD (though 1/3 of flow) is from industry. Figure 2.3 compares electricity use against BOD. Monthly Energy Use vs. Monthly Inlet BOD Daily Energy Use vs. Daily Inlet BOD 3, 16, Energy Use (kwh/month) 25, 2, 15, 1, y =.1156x R² =.7 Energy Use (kwh/day) 14, 12, 1, 8, 6, 4, y = -.342x R² =.369 5, 2, 1, 2, 3, 4, 5, 6, Inlet BOD (kg/month) 2, 4, 6, 8, 1, 12, 14, 16, Inlet BOD (kg/day) Figure Electricity Consumption vs. Biochemical Oxygen Demand (BOD) The data above highlights unexpected relationships between BOD and energy use for Bell Island Wastewater Treatment Plant. It is expected that the higher the BOD is, the higher the energy use is. However, only 4% of the daily variation in electricity consumption can be attributed to changes in BOD (as indicated by an R² value of 4%). The daily recordings are sporadic (i.e. are not recorded every day of the week). This would help to establish the relationship between BOD and energy use more accurately. The establishment of a benchmark for which to compare wastewater treatment productivity to energy use requires further investigation. NRSBU BISTP Level 2 Energy Audit 9

12 Figure 2.4 shows the hourly load and load duration curve for each time-of-use meter for the historical energy use period (February 211 January 212). The load for the Aeration Basin etc. meter is inconsistent but one-off peak loads are rare. The kva load duration pattern sees two sharp increases in kva with no corresponding changes to kw readings. This is likely to be due to one or two large items of equipment. The load for the Autothermal Thermophilic Aerobic Digesters (ATAD) etc. follows a seasonal pattern and a smooth load duration curve with the exception of the highest few loads. The Inlet, Outfall etc. meter had a poor power factor before June 211 and peak load reductions have resulted since its improvement. The peaks well above normal operation are likely to be the result of wet weather. Aeration Basin etc.: Load Aeration Basin etc.: Load Duration kva/kw kva/kw Feb 11 Mar 11 Apr 11 May 11 Jun 11 Jul 11 Aug 11 Sep 11 Percentile kw kva Oct 11 Nov 11 Dec 11 Jan Percentile kw kva ATAD Plant etc.: Load ATAD Plant etc.: Load Duration kva/kw kva/kw Feb 11 Mar 11 Apr 11 May 11 Jun 11 Jul 11 Aug 11 Sep 11 Percentile kw kva Oct 11 Nov 11 Dec 11 Jan Percentile kw kva Inlet, Outfall etc.: Load Inlet, Outfall etc.: Load Duration kva/kw kva/kw Feb 11 Mar 11 Apr 11 May 11 Jun 11 Jul 11 Aug 11 Sep 11 Percentile kw kva Oct 11 Nov 11 Dec 11 Jan Percentile kw kva Figure Load Duration Curve NRSBU BISTP Level 2 Energy Audit 1

13 2.3 Electricity Split Figure 2.5 shows the electric energy split for the entire process area. Energy splits provide the basis for audit calculations and are used to identify areas with high energy consumption and methods to reduce this. The electricity splits shown below have been estimated from data recorded during the site visits and from the motor list data provided. The processes which consume the most electricity are aeration (58%) and the Autothermal Thermophilic Aerobic Digestion plant (ATAD) (26%). Note: ATAD includes pumps and motors associated with each train of digesters. Pumps / motors at either end of the trains are included within sludge tanks / pumps. The review period used for the total electricity consumption and cost is the year ending January 212. Clarifiers.9% Ponds & Farmers Water 1.8% RAS/WAS DAF 2.% 2.7% Sludge tanks/ pumps 6.3% Preliminary Outfall.8%.6% Washwater.4% Other.7% ATAD 26.5% Aeration 57.7% Figure Electric Energy Split Figure 2.6 shows the annual cost split for the entire process area. DAF 2.7% RAS/WAS 1.9% Clarifiers 1.1% Ponds & Farmers Water 1.8% Sludge tanks/ pumps 6.3% Preliminary 1.% Outfall.7% Washwater.5% Other 1.2% ATAD 25.3% Aeration 56.8% Figure Electricity Cost Split NRSBU BISTP Level 2 Energy Audit 11

14 3 TARIFF REVIEW 3.1 Electricity Meridian Energy supplied electricity to Bell Island Wastewater Treatment Plant through multiple 11kV feeders via Network Tasman during the 12 months of energy data analysed (February 211 January 212). Copies of typical electricity bills for each tariff are shown in Appendix B. Bells Island Wastewater Treatment Plant has 3 time-of-use meters. These are: 48125NTFE5 (Aeration Basin, Dissolved Air Floatation, Return Activated Sludge, Waste Activated Sludge) 32543NT962 (Inlet, Screens, Outfall, Wash Water) 597NT746 (Autothermal Thermophilic Aerobic Digestion plant, (ATAD), Secondary Clarifier, Workshop, Sludge Tanks and Pumps) There is also a small fraction of energy use on non-half-hourly (NHH) metering. 5923NT-B86 (Dewatering Plant and Electrician s Workshop) (Pond Aerators, Farmers Water) Energy Charges For the time-of-use meters, electricity is purchased at a Fixed Price Variable Volume (FPVV) which divides the year into 144 time slots for time of day (4 hour slots), day of week (business days and non-business days) and month of year. Each time slot has a fixed hedge rate based on a number of variables related to the time of year. The average hedged rate for these meters for the year ending January 212 was 9.6 /kwh. (Note: Under the new electricity tariff in effect February 212, the average hedged rate will decrease to 8.7 /kwh for the year ending January 213 and 9.2 /kwh for the year ending January 214). The energy tariff for meters has the following components: Variable Charges: ATAD Plant etc. Aeration basin etc. Inlet, Outfall etc. Hedged Energy Charge 4-17 /kwh 4-17 /kwh 4-17 /kwh Nelson City Council Electricity Authority Charge Fixed Charges:.24 /kwh.24 /kwh.24 /kwh Data Reconciliation / Reading Fee $32 /month $32 /month $32 /month CT Monthly Fee (sewerage plant only) $15 /month TOTAL ENERGY COST $125,9 /year $24,1 /year $1,6 /year For ICP number 5923NT-B86 electricity is purchased on an anytime tariff. Variable Charges: Anytime Rate /kwh TOTAL ENERGY COST $78 /year NRSBU BISTP Level 2 Energy Audit 12

15 For ICP number electricity is purchased on a day/night tariff. Variable Charges: Day Rate (7 am 11 pm) 25.6 /kwh Night Rate (11 pm 7 am) /kwh TOTAL ENERGY COST $9,2 /year The energy prices used for savings calculations are listed below. Time-of-use rates (hedged energy charges), have been reduced compared to those for the analysed historical energy use period (Feb 211 Jan 212) due to Bell Island Sewerage Treatment Plant changing power suppliers from Meridian to Trustpower in February 212. ATAD Plant etc. and Aeration Basin etc. Inlet, Outfall etc. Non-Business Day Business Day Non-Business Day Business Day Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Average The anytime electricity rate for ICP number 5923NT-B86 is assumed to remain at /kwh throughout the year. The rates for ICP number are assumed to remain at 25.6 /kwh (day) and /kwh (night). Network Charges Bell Island Sewerage Treatment Plant has connections through the Network Tasman distribution network. Network Tasman bills Bell Island Sewerage Treatment Plant through the electricity retailer (Meridian Energy until February 212, Trustpower since). The following network charges are applicable to Bell Island Sewerage Treatment Plant. Variable Charges: ATAD Plant etc. Aeration basin etc. Inlet, Outfall etc. Winter RCPD Average Rate $.2272/kVA-day $.2272/kVA-day $.45/kVA-day Variable Distribution Charge /kwh /kwh /kwh Fixed Charges: Anytime Capacity Distribution $8,72 /year $24,18 /year $ /year TOTAL NETWORK COST $41, /year $94,2 /year $1,5 /year NRSBU BISTP Level 2 Energy Audit 13

16 The Winter Regional Coincident Peak Demand (Winter RCPD) Charge is based on the average of the 12 hourly peaks for each billed site co-incident with the highest peak demands experienced by Transpower. The amount charged stays constant on each meter for the year of energy use analysed (April 211 March 212). This equated to $.2272/kVA-day for the ATAD Plant etc. and Aeration Basin etc. and $.45 /kva-day for the Inlet / Outfall etc. The winter peak demand for billed sites is said by Network Tasman to regularly match the peak demand for Transpower due to weather, time of day etc. Therefore, sites that can move their peak demand to atypical times of the day should see a reduction in energy costs. As the load pattern for most of Bell Island Sewerage Treatment Plant is flat, a reduction in installed load should result in peak demand savings. The Anytime Capacity Distribution Charge equates to $.139 /kva-day. During the year April 211 March 212, the capacity considered to be required remained at the following for each of the sites: 172 kva ATAD Plant etc. 477 kva Aeration Basin etc. This capacity charge is billed based on the previous year of data, and is also independent of any site requirements for capacity (e.g. fuse sizes). Therefore, if the demand can be reduced, this charge can be reduced without capital expenditure on-site. For ICP number (Dewatering Plant), the capacity charge is $.52 /kva-day. The required capacity was determined to be 11 kva for the year from April 211 March 212. This charge equates to around 8% of the total annual cost of electricity for this meter. Given the very sparse energy use of this site, reducing this charge is the key to energy savings for this meter. Variable Charges: Capacity Charge $.52 /kva-day TOTAL NETWORK COST $2,465 /year The electricity bills for ICP number were not analysed. It is assumed for this meter there are no network charges and fixed costs are 96.5 /day. It is recommended the names of some meters be changed from those used on the Meridian electricity bills (if not done so already by Trustpower), as these are confusing. Names used by Meridian Energy (and proposed changes in brackets) are below: Sewerage Plant (Aeration Basin and DAF) Aeration Basin (Primary clarifier, inlet and outlet) New Sewerage ATAD Plant, Bell Island (Dewatering and Electrician s Workshop) No change is necessary for the main ATAD Plant meter. NRSBU BISTP Level 2 Energy Audit 14

17 4. PROCESS DESCRIPTION The Bell Island Sewage Treatment Plant (BISTP) processes 15, cubic metres of wastewater each day (on average). Sewage is transported through a network of underground pipes and pumps to BISTP. Pipe diameters increase in size as the water combines from pipe branches and approaches the treatment plant. Considerable infiltration of storm water occurs during heavy rains which can raise the hydraulic load of the plant by up to five times (13, m 3 for a dry day, 5, m 3 for a very wet day and 7, m 3 for a worst-case wet day). Weather conditions have a much greater impact on wastewater volume than the time of day, although there are peaks associated with each. Approximately 67% by volume of the sewage flow is from domestic sources (33% of biochemical oxygen demand (BOD) concentration), with the remaining 33% of flow (67% of BOD) coming from industrial and commercial sources. A site map with locations of major items of equipment is shown in Appendix A. Power ratings witnessed for pumps, motors etc. are included in Appendix C. NRSBU BISTP Level 2 Energy Audit 15

18 4.1 Preliminary Treatment Inflow and Screening: At the time of the site visit, the sewage entered Bell Island Wastewater Treatment Plant through two channels, the main 8 mm diameter plastic / polyurethane pipe from Nelson, and a 3 mm steel / polyurethane pipe from Mapua. There is also a 6 mm concrete pipe, although this is currently leaking. The leak has been found and the pipe will be repaired and used again. The inflow valve for the 6 mm pipe was appropriately closed during viewing due to not being in service. There are 2 screens to remove solid material larger than 3mm from the inlet sewage flow. The primary has a rated capacity of 76 litres / second per screen (2,73 m 3 / hour). The secondary screen is added to the process by means of a trip switch when the inlet sewage flow is above 76 L/second. Weir overflow makes the screen rotate and let water through, while the solids are held. The screened solids then pass through a classifier screw. The non-biodegradable solids are transferred into a skip. Flow meters are used to determine the fractions of total inflow that occur through the main pipes and the Mapua pipe. The classifier screw was seen to be on 24/7. Good practice is to operate this on a 2 minute cycle (for which the screen pump is typically on for around 5 minutes and off for 15 minutes), due to there often being few solids to screen. However, staff operate the classifier screw 24/7 as they had problems with the screw being blocked when operated sporadically. Therefore, changing classifier screw operation is not a savings opportunity. Grit Removal: Grit is removed early in the process to prevent damage to pumps / pipes further along in the process. The screened liquid flows via gravity to a grit arrester, which introduces compressed air to the water which is pumped to a grit classifier. This classifier forces dense (inorganic) matter to travel towards its centre and then discharges it to a skip for removal. The grit arrester is only on occasionally, which suggests it receives a signal to switch on given a level indication. This is good practice. The Fring aerators (at the Aeration Basin and the Autothermal Thermophilic Aerobic Digesters (ATAD)) can lose efficiency due to wear from sludge contents. This places importance on the solids screening and grit stages of the process. Wash water: Wash water pumps provide clean effluent to the grit system 24/7. This typically requires one of three pumps which run on a duty cycle. These pumps operate on a variable speed drive (VSD), which is good practice. There appears to be a submersible pump on the water supply. The load of this pump should be determined. NRSBU BISTP Level 2 Energy Audit 16

19 4.2 Primary Treatment Primary Clarifier The Primary Clarifier removes settle-able solids ( sinkers ) and floatable solids ( floaters ) from the wastewater. Waste water flows from the liquid output of the primary screen to the centre of the Primary Clarifier. Clean effluent flows out of the edge of each clarifier due to gravity and this flow is controlled via small weirs. The wash water pumps take the effluent overflow to the aeration basin (or to the grit system as necessary). Primary sludge is removed by a large scrape arm along the radius of the clarifier and paddles beneath which draw sludge to the centre of the clarifier. The two scrape arm motors are controlled together using a variable speed drive and operate at around 5% of full load. The Primary Clarifier is designed to allow bypass for the regulation of water if flows are very high and therefore energy savings (per m 3 of effulent) can be made by utilising this bypass as often as possible. The Primary Clarifier produces (and hence removes) 6-7% (7 tonne/day) of overall sludge production on site. The other 3-4% is removed in the Aeration Basin/ Secondary Clarifier, while the Nelson North sludge is imported. Aeration Basin: The Aeration Basin is positioned between the Primary and Secondary Clarifiers in the effluent flow sequence. The aeration basin is 5 metres deep and grows bugs for the treatment of solids using oxygen. The aeration basin accounts for 92% of the time-of-use meter 48125NTFE5. Flow through the aeration basin is said to peak 7: 1: and 22: 24: each day. The load profile of this meter is shown in Appendix C. The Aeration Basin contains 6 x 75 kw aerators and 5 x 3 kw aerators (6 kw in total), known as Fring aerators (being manufactured by Fring). A typical aerator is shown in Figure 4.2. Aeration operates 24/7, but due to three aerators being out of commission the facility is currently under-aerated. NRSBU BISTP Level 2 Energy Audit 17

20 Figure 4.2 Typical Aerator The aerators suck air and water through tubes, which is then dispersed through the outer prongs. The aeration load is higher during the day, thus requiring more oxygen. If oxygen is below intended levels, an odour event can result. The Fring aerators are rebuilt annually due to their arduous duty. Different parts of the Fring aerators are replaced at different times, as expensive parts are often retained for longer. This is also most likely due to the need to keep the plant operating at maximum capacity for all but a few hours. The annual schedule of maintenance seems to be a reasonable approach. The oxygen (O 2 ) level of the aeration basin is typically kept between.5 and 1.5 ppm, with an alarm being raised when O 2 is below.1 ppm, as at this level odour becomes a problem. During the site visit, the O 2 concentration was.7 ppm (with one 75 kw motor and two 3 kw motors not working). However, since the facultative ponds were receiving ample sunlight, a significant amount of aeration was being performed at the ponds. The aerators are controlled by dissolved oxygen (DO) levels. The Fring aerators operate with an efficiency of around.9 kg O 2 / kw. A diffused air system produces 1.24 kg O 2 / kw, thus using 27% less power for the same result. However installing diffused air is an expensive undertaking, costing around $3.2 million for the entire site compared to $.8 million for Fring motors. Air diffusion systems are currently not on the Nelson Regional Sewerage Business Unit plan. o Even if the diffused air system were only used at the aeration basin, and the 27% saving extended to demand as well as energy efficiency, the payback would be in the order of 3 years given: NRSBU BISTP Level 2 Energy Audit 18

21 An estimated cost for replacing aerators with diffused air at the aeration basin of $2.2 million. 27% savings on $278,/year (the estimated cost of running the aerators in the aeration basin) Therefore, this measure is not economically feasible while the Fring aerators are in good order. However, if at some stage in the future, the Fring aerators have to be replaced, then replacing those with a diffused air system needs to be re-examined as the payback period will be shorter. The Fring motors are 6-speed and are all attached to variable speed drives (VSDs). 5 aeration basin VSDs were read on site, the results of which are shown in Appendix C1. A 75 kw pump is assumed to use 43 kw on average, and a 3 kw pump is assumed to use 19 kw on average. Trickling filters are an alternative solution to the Aeration Basin for the removal of colloidal and dissolved solids (i.e. not settle-able or floatable). For an aeration basin, the removal of biochemical oxygen demand (BOD) from wastewater varies from 75-9 % depending on the amount of time the water is retained. Correspondingly, a low-rate or standard-rate trickling filter (assumed to apply to Bell Island Sewerage Treatment Plant), should remove from 8-85 % of the applied BOD. Therefore, the two approaches have similar BOD removal capabilities. It is recommended that trickling filters be re-examined if favourable information becomes available regarding improved BOD removal efficiency / reduced effluent duration when using a trickling filter. 4.3 Secondary Treatment Secondary Clarifier: The Secondary Clarifier receives gravity fed sludge /and water from the Aeration Basin. The Secondary Clarifier provides additional solids / liquids separation, using a similar configuration to the Primary Clarifier. After the Secondary Clarifier, the effluent can be transferred to the Facultative Ponds (F1, F2 and F3). Remaining sludge is separated into two streams, return activated sludge (RAS) and waste activated sludge (WAS), as detailed in Section 4.5. Whether liquid is transferred to WAS / RAS or to the ponds depends on the concentration of Mixed Liquor Suspended Solids (MLSS). If MLSS concentration is above 2 ppm (g/m 3 ), the liquid goes to WAS / RAS. If it is less than 2 ppm, the liquid goes to the ponds. On average, the waste activated sludge pumps receive 3-4 ppm solids from the secondary clarifier. Energy use of the Aeration Basin (etc.) time-of-use meter is plotted against MLSS in Figure 4.1. Note: This meter does not include the Secondary Clarifier, but is used in this comparison due to being highly energy intensive. NRSBU BISTP Level 2 Energy Audit 19

22 Daily Energy Use vs. MLSS Monthly Energy Use vs. MLSS 12, 25, 1, 2, Energy Use (kwh/day) 8, 6, 4, y =.726x R² =.199 Energy Use (kwh/month) 15, 1, y = -.781x R² =.4 2, 5, 5, 1, 15, 2, 25, 3, 35, 4, 45, MLSS(kg/day) 5, 1, 15, 2, 25, 3, MLSS(kg/month) Figure 4.1 Energy use vs. Mixed Liquor Suspended Solids (MLSS) Only 2% of the variation in Aeration Basin (etc.) electricity consumption can be attributed to changes in MLSS (as indicated by an R² value of 2%). There is almost no relationship between MLSS and Aeration Basin (etc.) energy use according to available monthly figures. Ponds: After secondary treatment, the treated wastewater is split between three facultative ponds (F1, F2 and F3 - formerly called oxidation ponds). These cover 3 hectares in total and are in parallel. Bacteria and nutrient in the waste promote vigorous growths of algae. During the day, near the surface, the algae generate oxygen by photosynthesis, further stabilising the wastes. The remaining solids settle to the bottom and are treated by anaerobic processes. Each pond has a 4 kw aerator. The water travels from these ponds to the Maturation Ponds (two in series M1 and M5). The status of the Facultative Ponds was read on two occasions during the site visit. Results were: Pond Total Flow Date and time Inflow l/s Dissolved oxygen (DO) concentration F1 8,78,335 15/5/12 11: /5/12 14: F2 7,46,24 15/5/12 11: /5/12 14: F3 1,136,5 15/5/12 11: /5/12 14: The ponds vary in DO concentration due to differing types of algae present, with F2 having significantly higher DO concentration than F1 or F3. Daylight savings settings on the SCADA system appeared to have been left on for the winter season with respect to the oxidation pond controls. This should be checked. Maturation Ponds: The two 1ha Maturation Ponds (in series) complete the stabilisation process and reduce bacteria numbers. They also provide storage capacity for intermittent release of the effluent. NRSBU BISTP Level 2 Energy Audit 2

23 Discharge: After an average retention time of 5-6 days (since primary inflow), the treated wastewater is discharged from the Maturation Ponds through a gravity driven outfall, into the Waimea Inlet. The treated effluent is typically discharged from high tide until 3 hours after high tide, so as to be taken out with the tide. For the most part the effluent discharge is gravity fed, although a discharge pump has been installed to increase the discharge capacity. The discharge pump is designed to operate during bouts of heavy rain as this is the driver for increased effluent throughput. During the audit process, the discharge pump was adjusted so that it only operates during high discharge flow (caused by high rainfall), with gravity-fed discharge flow becoming the default discharge strategy. Energy savings from this measure are discussed in EMO 2.1. The final effluent quality standards for discharge are shown in Table 4.2. The biochemical oxygen demand (BOD) and suspended solids standards are not as stringent as at North Shore Wastewater Treatment Plant (as an example). Table 4.2: Designed final effluent quality standards Pollution type Outlet Concentration (g/m 3 ) BISTP, Nelson North Shore BOD 5 <4 <1 Suspended Solids <1 <15 Total nitrogen 1-2 Ammonia & Ammonium <2 Pollution type Outlet Concentration (kg/day) Total nitrogen 5 Phosphorus 15 Oysters are known to collect on the discharge diffuser, although this area was not witnessed on site. Currently the approach is to clean the oysters off the discharge diffusers; however there may be ways to do this using ultrasonic technology. For example, boat hulls can be kept free of barnacles by using an ultrasonic device, thus improving the fuel efficiency the boat. Automating discharge diffuser cleaning would save maintenance costs rather than energy costs. 4.4 Solids Treatment Figure 4.3 shows the sludge throughflow for the three main components of digester sludge ever since all three were recorded on a day-to-day basis. Primary sludge accounts for just over 5% of the total sludge sent to the digesters (~63% of on-site sludge); Dissolved Air Flotation (DAF) sludge accounts for just over 3% (~37% of on-site sludge); Nelson North sludge accounts for just under 2% of the digester sludge. NRSBU BISTP Level 2 Energy Audit 21

24 6, 5, 4, 3, 2, 1, 12-Mar 19-Mar 26-Mar 2-Apr 9-Apr m3 Sludge 16-Apr 23-Apr 3-Apr 7-May 14-May 21-May FT638 (Primary Sludge) FT531 (DAF) FT511 (North Nelson) Figure Sludge Flow Meter Volume: 12 March May 212 Primary Sludge The sludge that settles and is removed within the primary clarifiers is called primary sludge. Sludge from the base of the clarifier is pumped to the digesters. The means of measuring sludge concentration at various stages of the process is desired but has not been planned or implemented yet. Primary sludge is currently pumped directly to the sludge storage tank (T22), although there is functionality on the SCADA (Supervisory Control and Data Acquisition) system to send primary sludge to the Gravity Belt Thickener or the Dissolved Air Floatation unit, or to turn the sludge pumping off. The primary clarifier sludge pumps (P618 and P624) operate when there is enough sludge. These pumps run most efficiently at higher speeds; therefore they are on a timed sequencer with one operating on duty and one standby. They can become worn down if the initial grit system lets grit through. This reduces their efficiency. Low density organic matter such as fats (referred to as scum ) float to the surface of the Primary Clarifier. These are scraped away to a scum tank, where they are pumped downwards so as to exit the clarifier at the same point as the settleable solids. Dissolved Air Floatation (DAF) unit In 25, the Dissolved Air Floatation (DAF) process was added as a gravity separation system that uses air bubbles in the wastewater holding tank to help float insoluble materials to the surface so they can be removed. The separation process is aided by dosing the effluent with polyelectrolytes. The polyelectrolytes are introduced to the process early for maximum contact / binding time (1 kg/m 3 of solids). The resulting flocculants cause these materials to join together in clusters that are lighter than water and therefore float. Based on site observations, the DAF is assumed to operate for 5 days a week, 24 hours a day on average, though this can vary from 3 days a week to 24/7, depending on other treatment plant loads. NRSBU BISTP Level 2 Energy Audit 22

25 Part of the clean effluent out of the Dissolved Air Floatation (DAF) unit is pressurised by two 15 kw pumps on VSDs, operating at about 5 kw each, and pumped into the compression tank. An air compressor supplies air at bar to the compression tank. Here the air is absorbed in the high pressure effluent water. This air saturated effluent water is discharged at the bottom of the DAF unit (below the main feed). Once the water pressure is reduced, the absorbed air releases in the form of fine bubbles, which carry sludge to the top of the DAF unit. The sludge is then scraped off the surface to a collection tank using a skimmer and the remaining underflow water is sent to the ponds. The sludge from the DAF is transported to the sludge holding tank. The installed maximum load of the DAF should be reviewed with regards to EMO 3.1. It is believed the Gravity Belt Thickener (GBT) would use less energy than the DAF to perform the same function (though at a greater polyelectrolyte cost). As mentioned in EMO 1.1, an asset register should be established that documents the energy use and intended operation of each item of process equipment. P56 (believed to be the DAF Feed Flow pump) was listed as having 196 Hz frequency on the SCADA (Supervisory Control And Data Acquisition) graphic for the secondary clarifier. This was read elsewhere to be 19.6 Hz. The SCADA graphic should be checked and adjusted accordingly. Return Activated Sludge (RAS) The return activated sludge system takes the sludge from the secondary clarifier and returns it to the Aeration Basin. This increases the contact time of the waste with the micro-organisms required for water treatment and is used to promote the growth of more micro-organisms. Two 11 kw pumps are used in this area (one on / one standby). This return activated sludge was seen to be operating at 74 litres/ second. The pump operation for return activated sludge is very efficient in that pumps operate for a length of time relative to the thickness of the blanket of sludge it is trying to move from the secondary clarifier. The following steps are used: Step Return Level (mm) Total pump min/hour (2 pumps) Waste Activated Sludge (WAS) Waste activated sludge is removed directly from the secondary clarifier and sent to the Dissolved Air Floatation (DAF) unit using the waste activated sludge pump. NRSBU BISTP Level 2 Energy Audit 23

26 This pump is controlled with a variable speed drive. The waste activated sludge was seen to be operating at 4.4 litres/ second (a small fraction of the return activated sludge flow). The waste activated sludge is about.6-1% solids when it reaches the Dissolved Air Floatation (DAF) unit. Solids Dewatering Building (SDB) There is currently provision for the primary solids to be sent to the Solids Dewatering Building where they can be processed for removal to landfill. The Solids Dewatering Building dewaters the digested sludge for bio-solids disposal. Several potential practical uses for digested bio-solids exist including soil regeneration, but these usages usually have to overcome political and health barriers. This facility is currently inactive due to the consistent and safe use of solids for fertilising trees on Rabbit Island. There is also an activated carbon filter external to the Solids Dewatering Building, which only operates when the Solids Dewatering Building is operating. The Solids Dewatering Building currently uses its own meter resulting in significant annual cost. This is discussed further in EMO 1.2. Gravity Belt Thickener (GBT) plant: The Gravity Belt Thickener is currently not in operation as the DAF has been seen to be sufficient for the separation of solids. It could act as an alternative to the DAF and is still set up for sludge separation. Like the DAF, the Gravity Belt Thickener is polymer dosed. It is said to have only been in operation for 2 months after which it was not required. During the site visit, it was noted the Gravity Belt Thickener could potentially use significantly less energy than the DAF to thicken sludge due to much reduced pumping equipment. This is discussed in EMO 2.3. Sludge Storage Tank: Sludge from the bottom of the primary clarifier (pumped by P618 and P624) and sludge from the DAF (pumped using a 7.5 kw worm pump ), go to the sludge storage tank. The Nelson North sludge tank receives (by truck) sludge unable to be used at the Nelson North water treatment plant. This is also pumped to the main sludge tank. The Nelson North sludge pump only operates when required to by Nelson North. This can be anywhere from once a day to twice a month. The power used by the associated pump should reflect this variance. The amount of sludge fed to the sludge storage tank is monitored carefully. The age of sludge that reaches the storage tank can vary from 1 to 3 days (longer in a few cases). Sludge is on average around 5 days old when it reaches the storage tank. Autothermal Thermophilic Aerobic Digesters (ATAD): The two-stage ATAD process uses heat released by microbial activity to achieve and sustain optimal temperature range for thermophilic digestion. The digesters are fed sludge from the sludge storage tank and deliver it to the Bio-solids tank to be pumped to Rabbit Island. Two pumps operating in duty/standby feed the pipes which split the sludge between the A, B and C trains. Each train has a valve and actuator. A heat exchanger was installed to pre-heat NRSBU BISTP Level 2 Energy Audit 24