SECTION C1 Description of the Existing Wastewater Treatment Plant The wastewater treatment plant in Rathvilly was constructed in 1954 on an inclined site approximately 350m west of the town centre. There is no electricity supply on site. A strip of land to accommodate a new access roadway to the site has recently been purchased by Carlow County Council. This new access roadway is parallel to and near the River Slaney and will provide access to the site from the N81 to the south of Rathvilly Bridge. This access roadway is fenced on both sides, and is provided with gates to facilitate access to surrounding fields by the landowner. The area of the original wastewater treatment plant site is 0.19 ha, and with the recent acquisition by Carlow County Council of adjacent land to accommodate the upgraded plant, the total site area has increased to approximately 0.39 ha. The existing wastewater treatment process comprises a traditional twin percolating filter system with primary treatment upstream in an Imhoff tank and downstream secondary settlement in humus tanks. The treatment system was originally designed to treat the wastewater from a population equivalent of 600 and is now receiving a higher estimated loading corresponding to a population equivalent in excess of 1,000. The system operates entirely on gravity flow and incorporates no pumping or electrical equipment. Raw wastewater from the town discharges to an inlet chamber at the head of the treatment plant via two 225 mm diameter gravity sewers (one from Ballyoliver and the other form the town) and a pumped main from the northern bank of the River For inspection purposes only. Slaney. Consent of copyright owner required for any other use. Flow gravitates from this chamber via a 300 mm diameter pipe to the preliminary treatment works, from where storm water overflows the storm weir at the inlet, and is discharged without undergoing any treatment via a 300 mm diameter sewer to the River Slaney. On the main process stream the flow splits and is passed through two parallel channels, each fitted with a manually raked coarse bar screen. The screenings are deposited on the ground alongside as there is no screenings container on site. Grit settles out in the screening channels and is manually removed. Downstream of the screening channels, flow gravitates to the Imhoff tank with a total plan area of approximately 18m2 and an upper chamber with an internal capacity of approximately 33 m3. Rathvilly Waste Water Works – Section C 1 EPA Export 26-07-2013:11:14:19 Settled sewage gravitates from the Imhoff tank to a chamber from where it is siphoned by two separate siphons (operating in parallel) to the two percolating filters. Sludge that settles in the bottom of the Imhoff tank is currently removed by a vacuum tanker, while in the past it was pumped manually to sludge drying beds on site. The walls of the percolating filters are constructed using large stones with a concrete capping and each has an internal diameter of approximately 9.3m and a bed depth of 1.68m. The filters are filled with stone media (approximately 122m3 each) and are fitted with rotating distribution arms that are driven by the force of the siphon feed. The apertures in these are cleaned manually. The effluent from the percolating filters gravitates to two humus tanks operating in parallel (total operational volume approximately 16.6m3). The effluent from these humus tanks then gravitates to an outlet manhole where flow combines with the storm water overflow (from the plant inlet) and discharges by gravity via 300mm diameter cast iron outfall into the nearby River Slaney. Settled sludge from the humus tanks is pumped to the sludge drying beds. There are three sludge beds on site, with a total plan area of 28m2 and these are filled and emptied on a sequential basis. The sludge liquor from the beds gravitates to the outlet manhole for discharge with the treated effluent and storm water, while the thickened sludge is taken to Tullow WWTP for dewatering. General In determining the optimum means of meeting the future demands of Rathvilly in terms of wastewater treatment and disposal, a stepwise approach was adopted. Once the future wastewater flows and loads for treatment had been determined it was necessary to identify a suitable water body into which the treated effluent could be For inspection purposes only. discharged. The beneficial Consentuses ofof copyright this ownerwater required body for any and other use.the legislative requirements governing it were consulted to determine its water quality objectives. This information, together with current water quality and flow date, was used to determine the quality and quantity of wastewater that could be discharged into it from the proposed wastewater treatment plant (WWTP). Once the treated effluent quality standards had been determined, the minimum level of treatment to be provided to the wastewater prior to discharge was established. Reference was also made to the Urban Waste Water Treatment Regulations S.I. No. 254 of 2001, which specify minimum levels of treatment to be provided for discharges from settlement depending on their size and their receiving waters. The types of wastewater treatment technology capable of providing the required type of treatment and achieving the treated effluent quality standard were identified and assessed. This assessment determined the site area necessary to accommodate the proposed WWTP. The proposed site for the WWTP was then evaluated. Rathvilly Waste Water Works – Section C 2 EPA Export 26-07-2013:11:14:19 The wastewater flow and load to be treated in the proposed Rathvilly WWTP in the design horizon year of 2031 is summarised below in terms of flow and BOD loading. Summary of Future (2031) Flows and Loads DWF BOD P.E. Ammonia Total N Phosphate (m3/day) (kg/day) (kg/d) (kg/d) (kg/d) 803.54 232.09 3,868 63.51 73.92 11.13 Options for Receiving Waters The treated effluent from the Rathvilly WWTP discharges directly into the River Slaney, west of the town. There is no other potential receiving water in the immediate vicinity of Rathvilly. To determine the suitability of the River Slaney to continue to accept treated effluent from the Rathvilly WWTP for the foreseeable future the assimilative capacity of the river at its 95 percentile flow rate is calculated. The 95 percentile flow has been recorded (bas on up to date data from the EPA website, September 2006) as 1.15 m3/s at station 12013 situated upstream of the WWTP discharge location and upstream of the abstraction point for Rathvilly Water Treatment Plant. The 95 percentile flow was used in the calculation of assimilative capacity in terms of BOD, ammonia, nitrate and suspended solids. For the calculation of the assimilative capacity of phosphorus, it is the median flow that is relevant in terms of compliance with the Water Quality Standards for Phosphorus Regulations 1998. The medial flow at station 12013 was found to be 3.78 m3/s (based on EPA monitoring data for the period 1976 – 2003). For inspection purposes only. Consent of copyright owner required for any other use. As part of the Carlow Water Supply Scheme Districts 1 & 3 it is intended to increase the overall quantity of water abstracted from the River Slaney from 8,139 m3/day (6,939 m3/d at Rathvilly and 1,200 m3/d at Tullow) to 19,200 m3/d (18,000 m3/d at Rathvilly and 1,200 m3/d at Tullow). While it is understood that the river flow will not be allowed to fall below its current recorded dry weather flow, it is anticipated that the increased abstraction will result in a reduction in the flow in the river at Rathvilly. For the purpose of calculating the future assimilative capacity of the river at Rathvilly the 95 percentile low and median flow figures have been reduced by 18,000 m3/d (0.208 m3/s) to allow for current and proposed abstraction at Rathvilly, since the existing flow monitoring point is upstream of the abstraction point. A 95 percentile flow (less water abstraction) of 0.942 m3/s corresponding to a river flow of 81,389 m3/d has therefore been adopted for use in calculating the figure assimilative capacity of the river at Rathvilly (for BOD, ammonia, nitrate and suspending solids). A median flow (less water abstraction) of 3.572 m3/s corresponding to a river flow of 308,621 m3/d has therefore been adopted for use in calculating the future assimilative capacity in terms of phosphorus. Rathvilly Waste Water Works – Section C 3 EPA Export 26-07-2013:11:14:19 In calculating the assimilative capacity of the river the maximum permissible concentrations in the river (based on the applicable legislative requirements) and the median background concentrations (at closest monitoring location upstream of the WWTP discharge) for the most recent data available i.e. 2006 were consulted. BOD5 In terms of establishing the maximum permissible BOD5 concentration in the river, reference was made to the Quality of Salmonid Waters Regulations 1998 (S.I. no 293 of 1998), and the water quality requirements for waters supporting freshwater pearl mussels. The Quality of Salmonid Waters Regulations set a concentration limit of 5 mg/l for BOD5. However, a lower concentration limit of 4mg/l is generally accepted as the criterion for satisfactory water quality (Parameters of Water Quality Interpretation and Standards, EPA, 2001). The most recent biological assessment (2004) of the River Slaney indicated a quality rate of Q4-5 upstream of the wastewater treatment plant. Under Part III of the First Schedule of the Water Quality Standards for Phosphorus Regulations, 1998 the maximum BOD5 concentration in river with a biological quality rating of Q4 should be less than 3mg/l.