GAIRLOCH WWTW: CAR/L/ APPLICATION FOR TECHNICAL VARIATION SUPPORTING INFORMATION

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1 GAIRLOCH WWTW: CAR/L/ APPLICATION FOR TECHNICAL VARIATION SUPPORTING INFORMATION 1

2 Introduction Scottish Water is working hard to always do the right thing for the environment and our customers in Scotland and we propose to amend the waste water treatment arrangements for the Gairloch catchment area while continuing to meet our environmental licence and compliance obligations. This paper provides additional information in support of our Application for Technical Variation of the discharge licence for the WWTW. Background Gairloch WWTW was brought into service in 2002 as part of a village -wide wastewater improvement scheme which saw a number of septic tank discharges and raw outfalls intercepted and transferred to the newly constructed WWTW. The technology chosen to provide treatment at Gairloch was a membrane bioreactor, a relatively innovative technology. This was selected as it provided a high level of treatment to the influent sewage in a much smaller footprint than conventional wastewater treatment processes. At that time, it was considered to be the most appropriate means to treat wastewater to a high standard in small communities, effectively providing both secondary and tertiary treatment to the effluent. The treatment is provided in two ways: Barrier treatment the very small pore size of the membranes prevents the pass through of material larger than the size of the pores, including bacterial components of wastewater Biological treatment and breakdown of the organic load provided by a biomass growing within the membrane system In terms of design capacity, based on the understanding of the catchment at that time, the works was licensed and hence designed to meet a continuous flow of 16 litres per second from a (future) contributing population of 2,122; which was based on an assessment of the existing residential population, tourists, and Local Plan provision for growth. In 2013 Scottish Water reviewed the design horizon of the WWTW in line with a reappraisal of actual and anticipated growth. This resulted in an application being made to SEPA to amend the design population to 1,100 and FFT to 8l/s. At this time the bacteriological standards on the licence were also removed as it was confirmed that there was no receiving water designation which required these standards for environmental protection. Since it was commissioned, operation of the system has proven to be challenging with a significant constraint at the works in being able to sustaining the designed Flow to Full Treatment, with intervention and maintenance far above what was previously anticipated. The technology faces two principal challenges: Sustaining the required wastewater flow ( FFT) - through the membrane system, with the pores in the membrane clogging more quickly than in the specification and a risk of wastewater backing up and spilling upstream of the treatment works 2

3 Sustaining the biomass - the living biological media that provides treatment by breaking down organic matter. As the combined sewage inflows are relatively weak there is insufficient concentration of organic material to consistently feed the biomass These treatment issues are further explored in the Technical Review document submitted in support of this application. Previous Application As we have stated previously, the existing membrane treatment process used at the works has proven un-reliable to operate and maintain. Initially it had been our intention to seek a Technical Variation to the discharge licence for Gairloch WWTW to allow for a septic tank treatment, discharged via an extended outfall and in 2016 an application was submitted to SEPA to this effect. Following designation of the two new Bathing Waters at Big Sand and Gairloch we revisited our proposals to assess compliance with Bathing Water Standards Sufficient/Good/Excellent). The supporting modelling work that was undertaken had identified that the standards would not be met under the discharge conditions of the original proposal. Consequently a revised proposal to include for a fine strainer ( disc filter), and disinfected effluent discharge ( Ultra Violet disinfection) to meet designated bathing water requirements i.e. disinfection of the effluent discharge will take place only during the bathing season ( 1st June 15th September) was submitted. Under this proposal, outwith the designated bathing the discharge would be of septic tank effluent only. Following further representation from the community to Scottish Ministers this application was formally withdrawn in April Revised Proposal Since the previous application was withdrawn, Scottish Water engaged m2 ( a joint venture between Mott MacDonald and Stantec bringing process and engineering consultancy expertise together) to undertake an independent technical review of the existing treatment process, and appraisal of options to meet the environmental requirements of the receiving waters and ensure the discharge from Gairloch WWTW did not impact on bathing water quality. The m2 Technical Report is submitted in support of this application. In light of the report received Scottish Water propose to proceed with the option for septic tank treatment with disc filter and UV disinfection. We have assessed the impact of the proposed changes to the treatment and extension of the outfall and confirmed that it will comply with the environmental designations (please see the attached supporting information capturing results from Anderson Marine Survey). Bathymetry has shown the co-ordinates of the existing end of pipe to be , Scottish Water proposes, as part of the current project, to extend the end of pipe 25m to , in order to meet the required dilutions for septic tank treatment. The choice 3

4 of this location is a result of the pre-application discussions following completion of the Anderson Marine Surveys. The new point of discharge is ~930m from the boundary of the designated Bathing Water at Big Sand. SEPA Regulatory Method WAT-RM-13 states that the discharge should not compromise the achievement of good standards in the Bathing Water, and also upholds the principle of no deterioration. Extrapolation of the data in the submitted modelling report shows that a discharge concentration of 35,000 e.coli/100ml will achieve the excellent bathing water standard at a distance of 892 meters from the outfall. This is the discharge limit that Scottish Water are seeking in our application. Scottish Water is confident that the septic tank and UV treatment will deliver the project objectives of a sustainable treatment solution with a lower carbon footprint that the existing WWTW, while meeting environmental objectives. The additional monitoring work we have undertaken demonstrates that our discharge from Gairloch WWTW does not impact on water quality within the boundaries of the designated Bathing Waters, and this is reflected in the process design. Additional Measures Concerns have been raised by the community on perceived loss of amenity in Loch Gairloch outwith the designated bathing season following a change to the treatment process. Whilst there is significant microbiological quality data from the first two bathing seasons following designation which show that both designated bathing waters achieved Excellent status, there is a lack of data for the period outwith the bathing season. Following further community engagement, and in agreement with the Community Stakeholder Group, Scottish Water proposes to undertake the following activities: undertake sampling of the designated bathing waters during the winter 2018/spring 2019 period while the existing treatment process remains in operation. provide UV disinfection of the effluent outwith the designated bathing season in the first full year following commissioning of the new treatment process. During this time sampling of the designated Bathing Waters will continue. In the following year the discharge outwith the designated bathing season will revert to septic tank effluent only. Again, sampling of the designated Bathing Waters will continue during this period. These activities, coupled with SEPA s ongoing monitoring of WWTW final effluent quality, will provide a robust data set for comparison of microbiological water quality under three different treatment scenarios. This data will be reviewed to understand how microbiological water quality varies, and the effectiveness of additional UV treatment of the septic tank effluent outwith the designated bathing season. In order to ensure consistency in the sampling, analytical, and reporting procedures for these sampling activities Scottish Water have commissioned SEPA to undertake this work on our behalf. 4

5 The outputs from this assessment will be shared with SEPA and our stakeholders. Operation and Maintenance of the existing WWTW Scottish Water is committed to maintain and operate the existing WWTW within existing licence conditions while we finalise design and undertake the construction and satisfactory commissioning of the proposed treatment process. In 2017 Scottish Water invested 80,000 to replace 400 individual membranes and undertook in-depth inspection of all our pumping and overflow assets to ensure there were no issues which could impact on the performance of the sewer network. Operation of the WWTW continues to be monitored and as far as possible all planned maintenance activities will be carried out ahead of the designated bathing season to minimise the risk to bathing water quality. Where performance drops below expected levels we will carry out remedial actions to ensure a return to normal operation as quickly as possible. Design Flows and Accommodating Growth The 2011 Census data for the Gairloch Settlement Zone is recorded as 1104 however the settlement zone extends beyond Gairloch itself, and not every property in the village is connected to the public sewerage system. The current resident population of Gairloch is estimated as Scottish Water s Annual Return for 2018 assesses the connected population for Gairloch WWTW as 1186, which is slightly higher than the current licence PE of This is broken down as 638 resident population a close correlation to the NRS estimate a commercial population equivalent of 253, and a tourist population equivalent of 295. However a recent verification exercise has confirmed that the connected PE is 1137 and it is this figure which has been used in the design assessment. The original COPA Consent application for the WWTW was based on a per capita contribution of 150 l/h/d ( G) and an assumed infiltration rate ( I) equivalent to 40% of population (P) x G, or 70l/h/d. This assumption of Infiltration was carried forward into the 2013 application mentioned previously, however analysis of 2016 and 2017 flow data has concluded that the infiltration rate is higher than this; equivalent to 100 l/h/d. On this basis the design flows are as follows: Dry Weather Flow (DWF) = PG + I + E (consented Trade Effluent) 1250x x m3/d, equivalent to 3.62l/s Flow to Full Treatment ( FFT) = 3DWF = 3PG + I + 3E 1250x150x x m3/d, equivalent to 7.96 l/s The proposed design of 1250 PE, or 8l/s FFT, gives design headroom of 113 PE which is equivalent to approximately 54 housing units. 1 NRS Small Area Population estimate ( SAPE) derived Settlement Populations - Highland 5

6 There have been Pre Development Enquiries for 8 properties as at end October In assessing future FFT an allowance of 3DWF has been made for future connections. However, all new connections require to be separate such that only foul flows are connected to the existing sewer network. Therefore the expected flows from all new properties will not be as high as has been allowed for in the design and there will be a slight increase in the design headroom as a result of this. 6