The Provision of First Time Sewage Treatment for the catchment at Laxey, Isle of Man

Transcription

1 The Provision of First Time Sewage Treatment for the catchment at Laxey, Isle of Man Briefing Document June 2017 Page 1 of 13

2 Contents 1 Introduction Background Information Laxey Drainage Catchment Hydraulic Model Coastal Dispersion Modelling Treatment Standards Isle of Man Bathing Water Standards Isle of Man Bathing Water Designation Relevance to this commission Likely Consent Parameters Site Location Site Constraints Environmental Impact Assessment Flood Risk Assessment Planning Public Consultations Treatment Process Scope of Works Terms and Conditions Appendix A Existing Storage Tank Outfall Location Plans Appendix B Bathing Water Quality results Page 2 of 13

3 1 Introduction Phase 1 of the Regional Sewage Treatment Strategy (RSTS) is now approaching its conclusion and attention is therefore refocussing on the settlements that have yet to see capital investment and with it significant environmental improvement and that include the RSTS Phase 2 project to provide first time sewage treatment facilities for Laxey. This Briefing document considers the requirement to construct a new Sewage Treatment Works (STW) to provide first time sewage treatment to this catchment. 2 Background Information 2.1 Laxey Laxey is located along the east coast of the Isle of Man approximately 8 kilometres to the north of Onchan. The town has been designated as a Service Village in the Strategic Plan for the Island and is unlikely to experience significant growth due to the topography of the area. There is a concentration of housing to the south of the harbour which extends, primarily along the seafront cliffs for approximately 1.5 kilometres. Development also extends inland along the Laxey Glen valley and also to the north of the Glen. The town is primarily residential, although there is a school and some non-residential premises in the harbour area. The majority of the town is served by a combined or partially combined sewerage system. The sewerage catchment is divided by the Laxey River, with approximately 50% of the catchment on the southern side of the river and 50% on the northern side. Most of the network operates by gravity; however, there is a small sewage pumping station that serves a housing estate on the southern boundary of the catchment and another pumping station that serves a small cluster of house near to the northern extents of the network. All sewage generated in the Laxey catchment gravitates to a tank constructed on the harbour side from where it is discharges in accordance with tidal conditions through an 18 cast iron outfall pipe discharging just outside the breakwater. This infrastructure was constructed in 1910 and a photo of the tanks location is shown below: 1910 holding tank at Laxey Page 3 of 13

4 The tank is still in operation albeit visually it is showing signs of distress e.g. cracking. Drawings of the tank and outfall arrangements are included at Appendix A. The settlement at Laxey has a domestic population of c.2000 with the usual small village accompaniments and in very broad terms it is thought that the overall population equivalent (PE) is of the order of 2, Drainage Catchment Hydraulic Model The drainage area catchment model is currently being updated and re-verified, and the flow surveys from this work will confirm population and flow projections. 2.3 Coastal Dispersion Modelling A coastal dispersion model is in the process of being procured and is likely to proceed in parallel to this commission. However based on previous studies undertaken at similar catchments at Ramsey and Peel, MU are of the opinion that the output will not differ greatly and will confirm that with a secondary treatment process constructed, compliance with the 1976 Bathing Water Directive would be achievable for the bathing waters at Laxey, and further through either the extension of the existing outfall and/or the installation and application of UV disinfection compliance with the 2006 BWD would be attained. In the interests of saving time on the overall programme it is on this basis that this particular commission is therefore proceeding now. (i.e. rather than waiting 6-9 months for the coastal dispersion modelling output). 3 Treatment Standards The settlement at Laxey has:- 1. A relatively small population equivalent 2. Discharges into coastal waters which have no special attributes that would deem them to be classed as sensitive waters. 3. The position of the actual outfall discharge pipe clarifies that it is not discharging into estuarine waters. Whilst the Isle of Man is a Crown Dependency with its own laws that are outside of UK legislation and further is not a member of the European Union, it has always tried to follow best practice and cognisant of all of the above and in accordance with the requirements of clause 7(b) of the Urban Waste Water Directive England & Wales 1994 (copy of relevant section below), it is believed that appropriate treatment should be provided at each location. (7) Appropriate treatment of urban waste water entering collecting systems shall be provided by 31st December 2005 in respect of (a) Discharges to freshwaters and estuaries from agglomerations with a population equivalent of less than 2,000; and (b) Discharges to coastal waters from agglomerations with a population equivalent of less than 10,000. (8) In this regulation (a) appropriate treatment means treatment of urban waste water by any process and/or disposal system which after discharge allows the receiving waters to meet the relevant quality objectives and the relevant provisions of the Directive and other Community Directives; 3.1 Isle of Man Bathing Water Standards The 1976 Bathing Water Directive was formally adopted by Tynwald in At the Tynwald sitting of March 2016 the following amended motion received unanimous support (20 votes to 3 in the Keys and 6 votes to 2 in the Council):- Page 4 of 13

5 That Tynwald is of the opinion that the EU Bathing Water Standards of 2006 should be considered as an objective to bring the Island into parity with neighbouring countries for the protection of public health and in particular for those who wish to use the Island s beaches and bathing waters for recreation. Therefore any solutions being proposed will have to conform to the requirements of the 1976 and the 2006 BWD. 3.2 Isle of Man Bathing Water Designation The DEFA website of the Isle of Man Government advises as follows:- The Government Laboratory tests seawater from 19 sites around the Island for compliance with the European Union Directive concerning bathing water quality. The sites that are monitored are not necessarily all bathing beaches in the sense of being beaches where people regularly swim, with facilities to support that activity, but they are beaches from which it is known that people do bathe or participate in sports that can involve or result in immersion and possible ingestion of water. 3.3 Relevance to this commission In brief there are no designated bathing waters in Laxey however it is known that people regularly swim and/or participate in water sports during the warmer months. For 20 weeks each summer season (mid-may to mid-september) the results of testing are plotted on a chart showing the quality of the water in a traffic-light manner as assessed to the present quality standard applicable in the Isle of Man (The EU Directive from 1976). This chart is displayed at the main bathing beaches and is updated each week during the season. Although it has not as yet been formally adopted here, assessment is also now made to the more recent 2006 EU Directive. The bathing water sampling results for Laxey for 2016 measured against both the 1976 and the 2006 directives standards are included at Appendix B. 3.4 Likely Consent Parameters For marine discharges of treated effluent the local regulator (Environmental Protection Unit (EPU) at DEFA) has always issued a consent to discharge licence with the following main determinant parameters: Biological Oxygen Demand (BoD): 40mg/l Suspended Solids (SS): 60mg/l There are no extenuating circumstances that would suggest this location would be any different and therefore the initial process quality parameters to be adhered to for the treated effluent are 40:60 (BoD:SS). Furthermore and cognisant that compliance with the 2006 BWD will also be required at the local bathing waters, it is envisaged that UV disinfection may be required as a tertiary treatment process. 4 Site Location Manx Utilities are currently in the process of procuring a parcel of land 2.5 acres in size directly adjacent and to the North of the existing storage tank. Consultation has been undertaken with the local Commissioners and Politicians who are supportive of the initiative to develop this site for sewage treatment purposes. A plan of this site is included at Appendix C. Page 5 of 13

6 4.1 Site Constraints The first and key observation to be made is that the site lies within a conservation area and as such any proposed development will need to give this due consideration and through the selection of appropriate plant and equipment and the use of carefully selected screening fit in and be sympathetic to the natural surroundings. It is noted that there are a number of private dwellings to the North and West of the land boundary all of whom are potential objectors to the development and therefore noise, odour, vibration and traffic are some of the issues that will need to be properly addressed. Currently there is no vehicular access to the existing storage tank and cognisant that future access (probably from Breeze Hill to the North of the site) would be required for operational and maintenance reasons it is an issue that will require detailed consideration. It should be noted that a public footpath six feet in width runs through the land in the position as shown on the plan. This will have to factored into the design of any new works so as this can continue to be utilised as such upon completion of the project Environmental Impact Assessment Cognisant of the site location and of the local village facilities including private dwellings in close proximity to the development site it is noted that an Environmental Impact Assessment study will need to be undertaken. It is thought the EIA would as a minimum be inclusive of the following fields (the list is not exhaustive): - Noise Assessment; - Vibration Assessment; - Odour Assessment; - Ecological Assessment; - Landscape Appraisal; - Traffic Assessment; and, - Plans and Policies Compliance Assessment Flood Risk Assessment A Flood risk assessment of the development site will need to be undertaken and the output and any recommendations considered and factored into the design solution Planning Planning permission will of course be needed for this development Public Consultations Public consultations through the local Commissioners will be an absolutely key activity during the design development and indeed during the planning period and whilst Manx Utilities will lead in this regard the Consulting Engineer will be required to provide support through the use of drawings, presentations, documentation and attendance at any Commissioner and public meetings. 5 Treatment Process Manx Utilities have utilised Integrated Rotating Biological Contactor (IRBC) Plants during the RSTS Phase 1 investment programme and this development seems suited to a similar approach insofar as the IRBC process offers a small footprint, low odour, low noise, low energy and low maintenance solution. The plant is generally partially buried, screened through the potential use of earthen bunds and considered landscaping and aesthetically offers much less of a visual impact than traditional activated sludge processes. There are numerous examples of sites developed during the RSTS phase 1 programme where IRBC s have been used and properly and adequately screened such that they blend into the natural environment and become almost inconspicuous. Therefore for this development it is envisaged that IRBC s would be used. Consultation will need to be undertaken with approved suppliers of IRBC plant during the design process. Page 6 of 13

7 It is envisaged that the UV disinfection treatment downstream of the secondary treatment plant would be applied through in pipe disinfection (rather than open channel) all contained within an underground chamber. NB this will need to be determined and optimised with the actual supplier of UV disinfection equipment. Consultation will be needed during the detailed design with suppliers for PLCs, HMIs, SCADA and variable speed inverter drives (VSD) and suppliers of systems integration (software design and supply and configuration of all necessary software including Mitsubishi hardware). 6 Scope of Works Manx Utilities are desirous to design and thereafter construct a new sewage treatment facility at Laxey and in doing so eliminate the discharge of raw untreated sewage to sea at this location. Manx Utilities are also cognisant that the discharge of treated effluent from such facilities should be such that there is no detrimental effects to the local bathing waters and that compliance with the 1976 and the 2006 BWD must be attained. Manx Utilities are therefore seeking to appoint a Consulting Engineer to progress this project through its outline design (procurement stage I1), through the detailed design (procurement stage I2) and thereafter to the completion of tender actions (procurement stage I3). Any post contract support (procurement stage I4) would be agreed as and when that milestone is achieved. It is noted that the successful consultant would also have been successful in applying to be listed within the new Consultancy Engineering Framework and as such would also be expected to undertake the role of Health & Safety Specialist for the project (as is now practised in the UK) and in parallel undertake the duties of the Planning Supervisor to accord with the local Isle of Man 2003 CDM regulations; details of which are expanded on in the actual Framework document. Support through the pre contract activities will be provided through the appointment of a Quantity Surveyor. The appointed Consulting Engineer must have a demonstrable track record in providing similar design solutions to include:- STW design including IRBC technology, MEICA design, including pumping station design, Road design It is believed that the Consulting Engineer s scope of works required to achieve the commission objectives are as follows:- 1. Consider the broad design concepts and initial site layouts 2. Consider the site access both during construction and thereafter during site operation 3. Consider the particular constraints 4. Develop an outline design for the whole works including the access 5. Consultation with the local Commissioners and local public at this milestone 6. Preparation of Procedure Notes Stage I1 Report 7. Undertake an Environmental Impact Assessment 8. Undertake a Flood Risk Assessment 9. Procure and manage all third party specialist surveys e.g. topographical and/or geotechnical 10. Develop the detailed design 11. Consultation with the Planning Authority 12. Consultation with the local Commissioners and local public at this milestone 13. Preparation of Planning documentation 14. Act as Employers Agent and provide requisite support during the Planning process 15. Preparation of Procedure Notes Stage I2 Report 16. Preparation of Tender Documentation * 17. Tender Actions* 18. Tender Evaluation * 19. Preparation of Procedure Notes Stage I3 Report Page 7 of 13

8 * Upon receipt of Planning permission 7 Terms and Conditions Option A Priced contract with activity schedule of the NEC3 Engineering and Construction Contract April 2013 terms and conditions shall apply. Page 8 of 13

9 8 Appendix A Existing Storage Tank Outfall Location Plans Please see the following plans provided separately: Laxey Outfall Plan Plan of the Cairns land parcel with footpath in blue June 2017 Page 9 of 13

10 9 Appendix B Bathing Water Quality results 2016 Page 10 of 13

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