BS-EN-12255-3-2000.pdf

| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | BRITISH STANDARD BS EN 12255-3:2000 The European Standard EN 12255-3:2000 has the status of a British Standard ICS 13.060.30 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW Wastewater treatment plants Ð Part 3: Preliminary treatment Licensed Copy: sheffieldun sheffieldun, na, Sat Oct 28 05:39:02 GMT+00:00 2006, Uncontrolled Copy, (c) BSI This British Standard, having been prepared under the direction of the Sector Committee for Building and Civil Engineering, was published under the authority of the Standards Committee and comes into effect on 15 December 2000 BSI 12-2000 ISBN 0 580 36451 8 BS EN 12255-3:2000 Amendments issued since publication Amd. No.DateComments National foreword This British Standard is the official English language version of EN 12255-3:2000. The UK participation in its preparation was entrusted by Technical Committee B/505, Wastewater engineering, to Subcommittee B/505/40, Wastewater treatment plants, which has the responsibility to: Ð aid enquirers to understand the text; Ð present to the responsible European committee any enquiries on the interpretation, or proposals for change, and keep the UK interests informed; Ð monitor related international and European developments and promulgate them in the UK. A list of organizations represented on this subcommittee can be obtained on request to its secretary. Further information regarding design and loading data can be found in the informative national annex. Cross-references The British Standards which implement international or European publications referred to in this document may be found in the BSI Standards Catalogue under the section entitled ªInternational Standards Correspondence Indexº, or by using the ªFindº facility of the BSI Standards Electronic Catalogue. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, the EN title page, pages 2 to 11 and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued. Licensed Copy: sheffieldun sheffieldun, na, Sat Oct 28 05:39:02 GMT+00:00 2006, Uncontrolled Copy, (c) BSI (8523($1?67$1$5 1250(?(8523e(11( (8523b,6 Part 3:Preliminary treatment; Part 4:Primary settlement; Part 5:Lagooning processes; Part 6:Activated sludge processes; Part 7:Biological fixed-film reactors; Part 8:Sludge treatment and storage; Part 9:Odour control and ventilation; Part 10:Safety principles; Part 11:General data; Part 12:Control and automation; Part 13:Chemical treatment; Part 14:Disinfection; Part 15:Measurement of the oxygen tranfer in clean water in activated sludge aeration tanks; Part 16:Physical (mechanical) filtration. NOTE For requirements on pumping installations at wastewater treatment plants and in their water feed field, provided initially as Part 2: Pumping installations for wastewater treatment plants, see EN 752-6, Drain and sewer systems outside buildings - Part 6: Pumping installations. The parts EN 12255-1, EN 12255-3 to EN 12255-8 and EN 12255-10 and EN 12255-11 became implemented together as a European package (Resolution BT 152/1998). The date of withdrawl (dow) of all conflicting national standards is 2001-12-31. Until the date of withdrawl is reached the National and the already published European Standards both co-exist. 1 Scope This part of this European Standard specifies the requirements for preliminary treatment of wastewater at wastewater treatment plants for over 50 PT. The primary application is for wastewater treatment plants designed for the treatment of domestic and municipal wastewater. Preliminary treatment may include one or more of the following options: ?screening; ?grit removal; ?grease separation; ?flow balancing and flow separation. Licensed Copy: sheffieldun sheffieldun, na, Sat Oct 28 05:39:02 GMT+00:00 2006, Uncontrolled Copy, (c) BSI Page 4 EN 12255-3:2000 ?%6,? NOTE Flow measurement and/or sampling provision may be included with any of the chosen options. Differences in wastewater treatment throughout Europe have led to a variety of systems being developed. This standard gives fundamental informations about the systems; this standard has not attempted to specify all available systems. Detailed information additional to that contained in this standard may be obtained by referring to the bibliograpy. 2 Normative references This European Standard incorporates by dated or undated references, provision from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this European Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies. EN 1085 Wastewater treatment Vocabulary. prEN 12255-1:1996 Wastewater treatment plants Part 1: General construction principles. prEN 12255-10:2000 Wastewater treatment plants Part 10: Safety principles. prEN 12255-11:1998 Wastewater treatment plants Part 11: General data. 3 Terms and definitions For the purposes of this European Standard the terms and definitions given in EN 1085 apply. 4 Requirements 4.1 General Preliminary treatment units are an important part of the overall wastewater treatment process, since they serve to ensure that the subsequent main treatment stages operate effectively. The preliminary process requires the removal of large floating and suspended solids, grit, grease and oil. Unless preliminary units are correctly designed, variations in flow may cause operational problems in the subsequent processes. This is particularly true of smaller plants. The type and size of units are influenced by the overall system and the nature of the wastewater to be treated. Combined sewerage systems usually have greater variations in flow than separate systems so that storm water separation arrangements and/or flow balancing may be necessary. Grit removal may be required to prevent damage to following treatment units and pumps. Where treatment plants receive significant amounts of organic industrial wastes, e.g. from food processing, the provision of grease and oil removal units is essential. 4.2 Planning The design considerations for a preliminary treatment system shall include a combination of the acceptable options to fulfil the process. Selection shall be based on an assessment of the characteristics of the wastewater to be treated, the size of the installation and its technical and economic consequences on subsequent processes. Licensed Copy: sheffieldun sheffieldun, na, Sat Oct 28 05:39:02 GMT+00:00 2006, Uncontrolled Copy, (c) BSI Page 5 EN 12255-3:2000 ?%6,? The requirements for the installation should be determined after consideration of the following factors: ?the effect of the flows on the subsequent treatment processes; ?the need to limit retention time in order to avoid septicity and/or premature sedimentation; ?fine screening and its influence upon sludge treatment and the necessity for primary treatment; ?grit removal with or without grease and oil separation; ?requirements for the disposal of grit and, the necessity for or possibility of grit washing; ?the reliability of the treatment process; ?the need to provide frost protection for exposed pipework and equipment where it is warranted. Once the requirements for the installation have been determined, consideration should be given to the requirements for the chosen site taking into account collection and disposal of screenings, grit, grease and oil. In addition, further general requirements are shown in prEN 12255-1:1996, prEN 12255-10:2000 and prEN 12255-11:1998. 4.3 Process 4.3.1 Screening When screening is selected as an option, the screens shall remove suspended solids and debris from the flow stream. NOTE 1 Screen aperture size may be influenced by the requirements of sludge disposal and subsequent process requirements or the discharge permit conditions for the final effluent. The following minimum apertures for screens are a guide: a) ? 20 mm to 50 mm :coarse screens protecting plant and to prevent flow blockage; NOTE 2 Coarse screens are used to protect mechanical plant by removing floating and large suspended solids in the form of paper, rags and plastics material. b) ? 10 mm to 20 mm :medium screens to prevent flow blockages; c) ? 2 mm to 10 mm :fine screens to reduce debris accumulating in the sludge. NOTE 3 Screens may be accompanied by washing, de-watering and compaction of screenings. The design velocities through screens shall not exceed 1,2 m/s at maximum flow. The velocity in the approach channel should not fall below 0,3 m/s at minimum flow. The potential health hazards which can arise from the handling and disposal of screenings shall be taken into account. See prEN 12255-10:2000 for further details. With the exception of overflows to a storm water holding tank where screening may not be necessary, wherever inlet overflows are installed screens shall be provided. Screenings shall be disposed of in accordance with national regulations. The normal configuration of screening installations is the multiple-channel type, a bypass with a hand-raked coarse screen being required for single-channel installations. Provision shall be made to ensure that each channel can be closed down separately. The structural design of the screen shall be capable of withstanding 0,5 m hydraulic head. Licensed Copy: sheffieldun sheffieldun, na, Sat Oct 28 05:39:02 GMT+00:00 2006, Uncontrolled Copy, (c) BSI Page 6 EN 12255-3:2000 ?%6,? The difference between upstream and downstream sewage levels shall be calculated for the maximum flow considering the aperture of the screen and the covering with retained material. The raking cycle should not exceed two minutes. Where this is not possible (e.g. deep installations) the design should accomodate higher loads, level differences and freeboard. The useful load of the mechanical rake shall be taken as 1 kN per m of screen width, with a minimum of 0,6 kN. The drive shall be designed for continuous operation and shall be appropriate for the largest solids expected. The scraping and driving elements shall be protected against mechanical and/or electrical overloading. Unless otherwise agreed, the design service life of the equipment for screening installations shall correspond to service life class 3, as specified in prEN 12255-1:1996. 4.3.2 Grit removal Grit removal units shall be designed to remove grit particles with a minimum diameter of 0,3 mm and a settling velocity of 0,03 m/s. A horizontal velocity of flow of 0,3 m/s is the optimum suitable for the removal of such grit particles in constant velocity channels and cross-flow settling basins. NOTE Combined sewerage systems contain significant amounts of grit which enter the sewers via road gullies and run off from paved areas. Separate sewerage systems may also contain grit, particularly in coastal or sandy areas. Separated grit may be washed and any organic matter removed should be returned to the flow for further treatment. The grit shall be disposed of in accordance with the health and safety requirements specified in prEN 12255-10:2000. Grit chambers should be constructed to operate in multiple channels. In the case of single channel installations, a bypass or alternative facility should be provided. Design features shall ensure that each channel may be taken out of service individually. Chambers shall be designed to withstand surge loads of grit as may occur during heavy rainfall and/or after long periods of drought. Pumps, scrapers and drives shall be designed accordingly. Scraper loading shall be assumed to be 10 kN/m 2 of the scraper, or 5 kN/m2 where the scraper adjusts automatically to the level of the material to be removed. Pumps shall be suitable in both design and material, for use in grit chambers (e.g. have a single channel or disk impeller). They shall be submersible and the suction should be adjustable in height whilst the pumps are in operation. Where air-lift pumps are used, they shall be provided with a backflushing facility. The velocity of compressed air in pipes of aerated grit chambers shall not exceed 20 m/s to prevent noise pollution. The construction principles of travelling bridges on grit chambers are specified in prEN 12255-1:2000. The design service life of drives, bridge scrapers and pumps shall correspond to class 3, that of air supply systems of air-lift pumps to class 2 and, that of air supply equipment for permanent aeration systems to class 3, all as specified in prEN 12255-1:2000. Licensed Copy: sheffieldun sheffieldun, na, Sat Oct 28 05:39:02 GMT+00:00 2006, Uncontrolled Copy, (c) BSI Page 7 EN 12255-3:2000 ?%6,? 4.3.3 Grease and oil separation Because of the deleterious effect of grease it shall be removed rather than being emulsified or solubilized. Where domestic and municipal wastewater includes discharges from hotels, restaurants and food processing operations, a grease and oil removal stage should be included in the design of the plant. NOTE 1As an alternative to grease removal as a separate stage, it may be possible to combine grease/oil and grit removal in a single unit or in the primary settlement stage. Where commercial kitchens or similar establishments are situated upstream of the plant, dedicated grease traps should be installed on the respective kitchen drains. Grease and oil removed from the wastewater shall be disposed of in accordance with the health and safety requirements of prEN 12255-10:2000. The design of the grease separator shall facilitate the safe and efficient removal of separable solids, grease and oil. NOTE 2Grease and oil removed from the wastewater may be de-watered before disposal. 4.3.4 Flow balancing and flow splitting Flow balancing may require flow measurement. Flows in excess of the design capacity of subsequent stages shall be diverted into flow balancing tanks. This shall take place after screening and grit removal where these processes are provided. The accuracy of flow measurement should not be significantly affected by the presence of suspended solids. NOTE 1Reduction in flow variations through the treatment processes may be beneficial but the cost and complexity of flow balancing is not easily justified on smaller plants. All flow balancing and flow splitting installations shall be designed