The Institute of Water Pollution Control NORTH EASTERN BRANCH 7 2 ItfiCS 1 PROCEEDINGS OF SYMPOSIUM "DESIGN AND OPERATION OF SMALL SEWAGE WORKS" held in the High Melton Training College, DONCASTER. 4th NOVEMBER, 1981. CONTENTS Opening Remarks by Mr. G. Eden, President of The Institute of j S IV Water Pollution Control. PAPER: «Design of Small Sewage-Treatment Works'. By K.D. Staples, F.I.C.E., F.I.P.H.E. (Member) Partner, Watson Hawksley A 1 - A18 Author's Introduction. A19 - A22 Discussion. A22 - A28 Reply to Discussion. A28 - A3I PAPER: 'Operation of Small Sewage-Treatment Works' By J.O'Neill, M.Sc., C.Chem., F.R.S.C. (Fellow) Yorkshire Water Authority. B 1 - B18 Author's Introduction. B19 - B22 Discussion. B22 - B3I Reply to Discussion. B31 - B37 PAPER: «The Use of Package Plants for the Treatment of Sewage from Small Communities' By. E.B. Pike, B.Sc, Ph.D., M. I.Biol., F.E. Mosey, B.Sc. (Affiliate), and D.W. Harrington, L.I.Biol. (Associate Member) Water Research Centre, Stevenage. C 1 - C36 Appendix I: A Guide to UK Manufacturers of Package Sewage-Treatment Plant. C37 - C4.9 Authors' Introduction. C50 - C53 Discussion. C53 - C59 Reply to Discussion. C59 - C62 PAPER: 'Sludge Treatment and Disposal at Small Sewage- Treatment Works' By G.P. Noone and A.K. Boyd Severn-Trent Water Authority. D 1 - D21 Authors' Introduction. D23 - D25 Discussion. P25 _ D30 Reply to Discussion. P30 _ 034 Chairman's Concluding Remarks (i) President's Concluding Remarks (ii) List of Exhibitors (iii) UBRAiW !ní6rna*io,r;s! fcsTorSii^ cvr.zr. to: SmriiüanS.-' Water Supply ERRATA 1. Page C2. ¿th line from bottom. For "110-120 l/hd of popalation" read "110-120 l/hd of population per day. 2« Pages C13 and C15. All references to organic loading rates of rotating biological contactors to read "g BOD/m d" instead of "g BOD/nr d". 3, Page C15. third line from bottom. For "... not at the loading disc stage..." read "... hot loading disc stages ...". ¿. Page C21. 13 lines from top. For "8.3 m" read "2.6 m (8.3 ft)". 5- Page C26. On line ¿, for "0.0¿3-O.3¿ kW/hd d» read n0.Q¿3-0.3¿ kWh/hd.d," On line 9 for "Nicholl" read »Nicoll». 6. Page D^t Hth line from top. Delete "bed" insert "land". OPENING REMARKS BY THE PRESIDENT. The PRESIDENT, Mr. George Eden, said that it was his pleasant duty on behalf of the Institute to welcome those present to this one day Symposium» He pointed out that the Institute, like many other organizations, was reconsidering its functions and one of the policies was to set in train a regular programme of symposia, both local ones organized by the Branches and national symposia organized centrally. This symposium was the first of the one day symposia organized by the North Eastern Branch of this Institute. The President then introduced the Chairmen for the day, Mr« W.S. Smith the Manager of the Peterborough Sewage Division, Anglian Water Authority, who was to chair the morning session, and Mr. J. Taylor, South Eastern Division Water Pollution Control Manager, Yorkshire water Authority, who was to chair the afternoon session. Mr« W.S. Smith, Senior Vice-President of the Institute, then took the chair and following some 'domestic' announcements introduced the firdt author of the day, Mr. K. Staples, and invited him to introduce his paper 'The Design of Small Sewage-Treatment Works'» DESIGN OF SMALL SEWAGE-TREATMENT WORKS By K.D. STAPLES, F.I.G.E., F.I.P.H.E. (Member) Partner, Watson Hawksley INTRODUCTION The design of small treatment works embraces several features which may be considered as special in comparison with the design of works of larger capacities. Firstly, the question of cost. Scale effects result in the capital cost of small treatment plants being higher per capita than larger plants. Land areas, the cost of land acquisition, access, electricity supply and general site development, welfare buildings etc., also tend to be high in proportion to the functional treatment units involved. These high-cost influences can only be countered by attention to simple and economic construction and equipment design. The second, and perhaps most important, special consideration is the question of works1 operation. The cost of manned operation has increased significantly in recent years, and in most costs small plant operation is high in comparison with the larger works. Obviously it is not appropriate to man small treatment units continuously, and the design challenge is therefore to build a plant which will function effectively and reliably with the minimum attention. EFFLUENT STANDARDS AND PROCESSES Effluent standards will usually be set in the discharge consent, but one may speculate that the Royal Commission Standard will often remain a most useful guide. Economy and convenience will indicate frequently that a 30 : 20 standard is not inappropriate since a more relaxed standard is not always consistent with reliable operation, and a more stringent standard also more expensive and difficult to maintain. The Royal Commission effluent standard was based upon a minimum dilution of 8 : 1, and small plants frequently suffer from the difficulty that the effluent is either discharged to soakaways or to a small stream or ditch, which dries up in summer so the effluent is again being absorbed into the groundwater system. In these systems of groundwater recharge, it will be A 1 evident, to avoid clogging the ground pores, that the removal of fine suspended solids from the effluent is probably more important than reducing the BOD. Whatever the standard set, monitoring may not be strict and the real performance criteria may be the absence of nuisance or complaint. The mass transfer and physical design parameters applicable to larger treatment units apply, but it is usually appropriate to adopt lower loadings and slightly larger units, especially for very small works, in order to deal with flow surges and to cater for the intermittent nature of the operational maintenance. Well-proven processes are available and should be used, although their application and engineering may have to be adapted. There is the normal need for twin units in order to provide a standby flow path whilst the individual tank is being cleaned or maintained, and the need for standby mechanical plant is accentuated by the planned infrequency of operational visiting. For all units there are advantages in ensuring that in the event of blockage, mechanical or electrical failure bypassing or emergency overflow is possible. Obviously good design will permit some degree of treatment to continue. FLOWS AND PUMPING The assessment of average dry-weather flows as a basis of design is normally relatively simple, and although local conditions will need to be checked an allowance of 150 l/hd/d can be considered normal for a domestic sewage installation Stormwater or groundwater entering the sewer system can lead to real difficulties and all small works should be based on a strictly separate system. In spite of separation flow multiples will be high, and the resulting difficulties are often referred to as 'surging1. A simple calculation will demonstrate that for a single ordinary household flushing of a toilet will result in a flow peaking factor of about 30, while a 2 3 bath emptying can double this factor ' . There are many graphical analyses of sewage peaking factors to show the attenuation effects with increasing populations and these usually suggest peak multiples of between 5 and 10 for population of 1000. For very small plants the peak flows are often of short duration and their surge effect may be balanced to some extent within the treatment process. In practice capacity for balancing can be achieved in the overall capacity of the first main units, or by purpose-built inlet arrangement with some A 2 restriction on rate of discharge to the tanks. Frequently sewage pumping is inevitable either at the site of the treatment works or offsite to lift the flows to the works, although effluent pumping is preferable to sewage pumping if this can be achieved. The need for pipes of adequate size to prevent blockages is valid for all works and the use of gravity sewers and sludge pipelines under 150 mm is to be avoided. Similarly for sewage and sludge the use of unchokable pumps is appropriate. This type of pump, taken with the need to have self-cleansing velocities in rising mains above about 100 mm in daimeter, inevitably means that flows will reach the works at high multiples and sometimes infrequently. The design of pumping stations and balancing arrangements for small works will require a compromise between long retentions and infrequent operation during periods of low flow and maximum permissible number of pump starts per hour. For a 100-mm pumping main or feed pipe served by a single operating pump at self- cleansing velocities the minimum possible capacity will amount to a peaking factor of about 4500 for a single person, more practically to cater for 100 persons a pumping main will deliver flow at a multiple of 45 DWF. SELECTION OF PROCESS The choice of treatment process will always be influenced by locai factors, not the least being site configuration and the available head across the site. Of equal importance is the availability of power, and the extent to which it may be appropriate to trade off a higher power consumption mechanical equipment system with what might be a higher land usage gravitational system. The choices are therefore strongly influenced by the local circumstances and the size of plant, and a subjective review of the principal options is given in Fig.1.