Water Quality Assessment V2.0.Pdf
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Water Quality Assessment A.1 Introduction The increased discharge of effluent due to an increase in the population served by a WwTW may impact on the quality of the receiving water. The Water Framework Directive (WFD) does not allow a watercourse to deteriorate from its current class (either water body or element class). It is EA policy to model the impact of increasing effluent volumes on the receiving watercourse. Where the scale of development is such that a deterioration is predicted, a new Environmental Permit (EP) may be required for the WwTW to improve the quality of the final effluent, so that the extra pollution load will not result in a deterioration in the water quality of the watercourse. This is known as "no deterioration" or "load standstill". The Environment Agency operational instruction “Water Quality Planning: no deterioration and the Water Framework Directive”1 sets out a hierarchy for how the no-deterioration requirements should be implemented on inland waters: deterioration exceeds 10% of existing water quality immediately downstream of the discharge. This 10% target is included, not because it is a statutory requirement of the WFD, but to ensure that all the environmental capacity is not taken up by one phase of development and there is sufficient capacity for future growth. Deterioration beyond the class limit immediately downstream of the discharge. For example, a watercourse currently in the Moderate class for ammonia moving to Poor. For the purpose of this WCS, these two methods for determining deterioration immediately downstream of the point of discharge have been applied, based on the principle that the WCS should, where possible, guide development to locations where it will not lead to environmental deterioration or require investment with a low cost-benefit being required to prevent deterioration. If the receiving watercourse is classed as "Bad" then no further deterioration is permitted, and, again within this study, this rule has also been applied immediately downstream of the point of discharge. EA policy allows alternative definitions of deterioration to be applied where it can be demonstrated that meeting the above criteria would be technically infeasible or where the costs of achieving no- deterioration is disproportionate to the cost. These alternative definitions of deterioration include allowing up to 15% of the water body or 1.5 km of classified watercourse length (whichever is the lower amount) to be in worse condition than the overall status, and assessing class limit deterioration at a downstream monitoring point. Application of these alternative approaches was considered to be beyond the scope of this WCS. It is, however, anticipated that these alternative approaches may be applied by the sewerage undertakers in their negotiations with the Environment Agency to determine future consents. Anglian Water and Thames Water prepared a RAG analysis of the capacity and performance of all WwTWs within Aylesbury Vale which may see increased flows due to housing allocations. This analysis identified 17 WwTWs with potential future capacity issues due to growth. Aylesbury Vale requested that a water quality impact assessment should be carried out at these 15 WwTWs: Anglian Water Buckingham Great Horwood Ivinghoe Padbury Stanbridgeford Steeple Claydon Whaddon Wing Cublington Winslow Thames Water 1 Environment Agency (2012) Water Quality Planning: no deterioration and the Water Framework Directive. Accessed online at http://www.fwr.org/WQreg/Appendices/No_deterioration_and_the_WFD_50_12.pdf on 03/10/2016 2016s3990 - Water Quality Assessment v2.0 1 Aylesbury Haddenham Long Crendon Stone Waddesdon Worminghall Note that Brackley and Cotton Valley WwTWs were excluded in this assessment as they are located outside of Aylesbury Vale, and both will receive the vast majority of their growth from South Northamptonshire and Milton Keynes respectively. This WCS assesses the potential water quality impacts due to growth in WwTW effluent flows and loads at the WwTW discharge points. Please note that, whilst the other WwTWs not considered in this assessment may have capacity within their consents to accommodate the planned growth scenarios, this does not necessarily imply that the watercourse would, with the existing consent, be able to meet Good Status. Additionally, it does not imply that future increases in discharges within the permitted consent would not lead to a deterioration occurring. A.2 Standards The WFD targets for Good Ecological Status (GES) for Biological Oxygen Demand (BOD), Ammonia (NH4) and Phosphorus (P) set by the EA for lowland and high alkalinity water bodies are shown in Table 1 below. Table 1: WFD targets for lowland and high alkalinity water bodies Determinand Statistic Target BOD 90 percentile 5gm/l NH4 90 percentile 0.6mg/l P Mean reach specific The EA has provided WFD 2015 set catchment/reach specific targets for phosphorus. On this basis the following targets (see Table 2) have been used at the WwTW discharge points assessed: Table 2: Phosphorus targets for "Good" status by WwTW WwTW P mean mg/l Waterbody/ WQ point Buckingham 0.08 GB105033037920 Great Horwood 0.076 GB105033038200 Ivinghoe 0.073 GB105033030490 Padbury 0.077 GB105033038220 Stanbridgeford 0.12 GB105033030530 Steeple Claydon 0.073 GB105033038220 Whaddon 0.08 GB105033037870 Wing Cublington 0.073 GB105033030540 Winslow 0.076 GB105033030570 Aylesbury 0.066 GB106039030410 Haddenham 0.081 GB106039030270 Long Crendon 0.066 GB106039030410 Stone 0.081 GB106039030270 Waddesdon 0.08 GB106039030420 Worminghall 0.081 GB106039030340 A.3 Methodology The contaminants assessed were Biological Oxygen Demand (BOD), Ammonia (NH4) and Phosphorus (P). 2016s3990 - Water Quality Assessment v2.0 2 The selected approach was to use the EA River Quality Planning (RQP) tool in conjunction with their recommended guidance documents. "Water Quality Planning: no deterioration and the Water Framework Directive" and "Horizontal Guidance". This uses a steady-state Monte Carlo Mass Balance approach where flows and water quality are sampled from modelled distributions based on data where available. The data required to run the RQP software were: Upstream river data: Mean flow 95% exceedance flow Mean for each contaminants Standard deviation for each contaminant Discharge data: Mean flow Standard deviation for the flow Mean for each contaminant River quality target data: No deterioration target 'Good status' target The above data inputs should be based on observations where available. In the absence of observed data EA guidance require that the following values were used: Flow mean: 1.25*DWF Flow SD: 1/3*mean Quality data: permit values or assumed values recommended by the Environment Agency If observed river flows were not available these were obtained from an existing model or a low-flows estimation software. If observed water quality data were not available these were obtained from an existing model or a neighbouring catchment with similar characteristics, or the mid-point of the WFD class. The observed data available for WwTWs discharges were analysed in the statistical tool, Aardvark and the values reported as "less than" (these are samples where it was not possible to get an accurate value and a limit value was assigned) were multiplied by 0.5 as agreed with the EA. 2016s3990 - Water Quality Assessment v2.0 3 A.4 Study Objectives RQP models were required to be set up and run using the present-day and a single future scenario representing development of all draft site locations (housing and employment) which would drain to each WwTW as reported in Table 3 below. Table 3: present day and future scenarios to model Mean DWF (Ml/d) WwTW Future growth Percentage Present day Future growth Total Change Buckingham 4.63 0.713 5.34 115% Great Horwood 0.54 1.537 2.08 385% Ivinghoe 1.33 0.106 1.44 108% Padbury 0.36 0.023 0.38 106% Steeple Claydon 0.951 0.059 1.01 106% Whaddon 0.214 0.651 0.86 404% Wing Cublington 0.85 0.080 0.93 109% Winslow 1.77 0.335 2.11 119% Stanbridgeford 3.10 0.057 3.16 102% Aylesbury 24.11 5.191 29.30 122% Haddenham 2.05 1.826 3.88 189% Long Crendon 0.58 0.076 0.66 113% Stone 0.99 0.075 1.06 108% Waddesdon 0.658 0.098 0.76 115% Worminghall 0.76 0.057 0.82 107% The study was required to consider increase to effluent flows as a result of development in each settlement to assess the impact of the increased contaminant loads on the receiving watercourses. These results were required to assess the potential impact on the watercourse which could cause the failure of one of these targets: Good Ecological status (GES), no more than 10% deterioration and no class deterioration. Where a WCS is predicted to lead to a WFD class deterioration, or a deterioration of greater than 10%, or a 'good status' failure it is necessary to determine a possible future permit value which would prevent a class deterioration using the RQP function tool that calculates the required discharge quality according to the specific river target. Where failure was predicted for any of the other scenarios, and the present-day upstream river quality did not achieve 'good status', the model was re-run for present-day and future effluent flows assuming that the river had 'good status', in order to assess whether the planned growth could prevent the watercourse from meeting GES in the future. When a new consent value was calculated, due to a target failure, this was compared against the effluent quality that can be achieved using Best Available Technology (BAT). The EA advised the following permit values are achievable using Best Available Technology and that these values should be used for modelling all WwTW potential capacity irrespective of the existing treatment technology and size of the works: BOD (95%ile) = 5 mg/l Ammonia (95%ile) = 1mg/l Phosphorus (mean) = 0.5 mg/l Note that phosphorus removal is the subject of ongoing national trials investigating novel techniques and optimisation of existing methods.