SEWAGE TREATMENT PLANT

Abstract: A sewage treatment plant for use aboard ship comprises a vacuum-assisted fresh water flush system delivering sewage to a collection tank (2). Sewage is collected from the collection tank (2) in batches for treatment. The sewage first enters a settlement tank (201) for removal of solids and is then transferred to an aeration tank (6). After aeration treated sewage is then transferred out in batches. The batch processing overcomes the difficulties of operating a continuous flow arrangement on a moving platform where liquid can move back and forth which presents difficulties in controlling the level of biological activity. A controlled amount of sewage transferred to the aeration tank is dilluted with grey water (206). A sludge storage tank (23) is provided and a suspended solids density probes are provided in the aeration and settlement tanks such that sludge is automatically withdrawn from these tanks to the storage tank to maintain the level of suspended solids within predetermined limits. The sludge storage tank is provided with aerators (35) and its temperature is controlled (37, 38) such that aerobic digestion renders the sludge inert. Small batches of aerated liquid are drawn from the aeration tank (6) into a chlorination tank (30) via a separation system (131) at controlled intervals between successive sewage batch transfers from the collection tank (2) to the aeration tank (6) via the settling tank (201) prior to discharge from the plant.

Description: SEWAGE TREAIMEM 1 PLAI ~ r The invention relates to the treatment of sewage and in particular, though not exclusively, to sewage treatment for use on amobileplatformsuch as a ship. Disposal of sewage from ships alongside or in coastal waters is a considerable current problem. Some harbours have sewage collection facilities, however, thesearenotalways available. I us sewagecollection and treatment plant is requiredwhereportfacilities areunavailable. Such plant is desirably highly efficient such that a small plantwill allow a ship to remain alongside for a long period. Currently there are three types of sewage treatment plant in common use. These are: a) Collect, hold and transfer (CHT) with either gravity or vacuum collection systems. b) Solid separation treatment plant (SSTPS) . c) Biological sewage treatment plants (BSTPS) . Biological Sewage Treatment Plants Theoretically, this type of plant should .allow ships to operate for long periods without the need for shore reception facilities. In practice they have not operated satisfactorily for the following reasons: a) The process depends upon effective reduction of the sewage by bacteria in the main aeration tank. This process is very sensitive to: (1) Oranges in the salinity of the flush water. (2) High hydraulic loads caused by defective toilet flushing valves or leaking tank cleaning spray cross connection valves. (3) Variations in the load cycle imposed by the operating pattern of the ship. Low loads are particularly detrimental as the bacterial population necessary for aerobic digestion of the sewage cannot be sustained. (4) Excessive use of detergents and cleaning agentswhich destroy the bacterial population. b) Being continuous flow processes there is no control over the residence time in each tank. This can result in incomplete processing of the sewage and carry over of solids. c) Ship motions affect the settling process. Solid Separation Treatment Plants This type of plant has the advantage that it is available on demand and can be shut down when the ship is outside controlled waters. However, theseplants have notbeenentirelysatisfactory inserviceforthefollowing reasons: a) Thelevel of biological activity of thedischarge does notmeet the standard expected to be set by international regulations. b) The filters have proved to be particularly troublesome. They block frequently .and require considerable and unpleasant maintenance. c) The plants have a relatively short holding time of only 5-7 days. d) The plants use a lot of disinfectant (151bs/day) . Collect, Hold and Transfer Systems The majority of these fitted in the Royal Navy are of the gravity collecttype.and are fittedtosmaller ships. Gravitycollection reducesthe plant size since flush water is minimised. The main problemwith CHT .systems is their limited hold time of 5-7 days. This in turn leads to the requirement for costly sewage reception facilities in the Naval Bases and can lead to problems when ships visit or operate in clean areas. The object of thepresent invention is toprovide .an improvedsewage treatment plantr capable of operation on a mobile platform and of use for longer periods than currently available plant. The invention provides a sewage treatment plant comprising a collection tank, a settlement tank, an aeration tank, transfer means and a transfer controller arranged such that sewage is transferred in controlled batches into the aeration tank and subsequently out after separation of solids whereby processing can be accurately controlled. Advantageously a fresh water flush vacuum system is used to collect sewage in the collection tank. The use of a small amount of fresh water, vacuum-assisted, eliminates difficulties experienced with the use of sea water of varying salinity. Thebatchprocessing overcomes thedifficulties of operating a continuous flow arrangement on amovingplatformwhere liquid can move back and forth. Known biological settling systems operate effectively only under static conditions because of difficulties in controlling the level of biological activity. In the present invention, a controlled amount of sewage is transferred to the aeration tank via a grey water dillution system for a controlled period before separation of the solids transfer from the aeration tank. The εettlment tank is provided between the collection tank and the aeration tank whereby sludge may be removed prior to aeration treatment and dillution with the grey water. Advantageously a sludge storage tank is provided and suspended solids density probes are provided in the aeration and settlment tanks such that sludge is automatically withdrawn fromthe aeration tank to the storage tank tomaintain the level of suspended solidswithin apredetermined limit. The sludge storage tank is provided with aerators and its temperature is controlled such that aerobic digestion renders the sludge inert. Advantageously the controller and transfer means are arranged such that small batches of the liquid are drawn from the aeration tank into a chlorination tank via a separation system at controlled intervals between successive sewage batch transfers from the collection tank to the aeration tank via the settling tank. The separation .system advantageously comprises a separator selected from the groupof tiltedplate separators, air floation and centrifuge. The arrangement is controlled such that a batch of liquid is transferred fromthevacuumseparation tank tothechlorinationtankpriorto entry of fresh liquid to the separation tank. The invention will now be described by way of example only with reference to the accompanying Drawingswhich shows a flow diagramof asewage treatment plant according to the invention. In a marine sewage treatment plant as shown in the Figure, sewage collected under vacuum is delivered via a conduit 1 to a 2000 litre sewage reception tank 2, a capacity equivalent to about twice the expected daily sewage collection for ahundredmanplant. Freshwaterisusedforflushing. Vacuum collection requires much less volume of flushwater and so leads to a more compact treatment plant than using conventional flush arrangements. TWo level probes 3 and 4 in the sewage reception tank 2 define the upper and lower limits for storage of sewage. Six hourly batches of sewage (2501itres) from the reception tank 2 are first passed via a pump and macerator 7 and a otorised valve 8 to a settl enttank201wheretheyaremixedwithquantitiesofsettlingagentfrom a polyelectrolyte tank 202 by a pump dispenser 203. Sludge from the settlmenttank 201 is transferred throughvalve204, togetherwith thesludge froman aeration tank 6, toasludgestoragetank23. Atsixhourly intervals liquid remaining (200 litres) in the settlment tank 201 is transferred via line5 bypump205 to the10,000 litres aeration tank6. Thusbulk solids are removed fromthe settlment tank201prior to aeration treatment. The sewage effluent tobe treated in the aeration tank is dillutedwithgreywater in the ratio15 parts ofwater to1partofsewage inorder tooptimisethebiological process. Greywater iswastewaterwhich is not classified as sewagefromeg showers, washing and the ship's galley. The greywater is connected to the sewage line 5 via a conduit 206. Every six hours a250 litrebatch ispumped along the line 5 to the aeration tank 6. Conditions in the aeration tank 6 are critical and these are monitored by various probes 15-20 whichmeasure temperature (15), dissolved oxygen (16), liquidlevels (17-19) andsuspendedsolids (20) . Whenthelevel of suspended solids reaches a critical value, sludge is automatically withdrawnthroughvalve21 fromthebase of theaeration tank6byapump22to an aerobic sludge digestion/storage tank 23. The sewage is aerated by air provided to fine bubble aerators 11 via air line 9 from a blower 10. After an initial period 200 litrebatches ofwell aeratedliquidare withdrawn hourly from the aeration tank 6 through a suctionhead12 byapump 13 through the separation system131 andavalve24 toachlorinationtank30. The separation system 131 comprises a centrifuge separator although tilted plateseparatorsoranair floatationseparatormaybeused. Betweenbatches the flew from the separation system 131 is diverted by valve 24 back to the aeration tank. Solids are returned from the separation system131 via line 14 to theaerationtank 6. The aeratedliquidbatches are transfer edtothe chlorination tank 30 through a chlorinationunit 31. The liquid remains in the chlorination tank 30 for four hours to ensure complete chlorination before it is discharged in 200 litre batches from the shipby mecins of a pump 32. Theliquidlevel in thechlorination tank 30 ismonitoredby level probes 33 and 34. Sludge which is transferred to the sludge storage tank 23 is aerated by aerators 35 (35 and 352) connected via valve 36 to the blower 10. The aerators 352 are fine bubble aerators. The sludge is kept at a controlled temperature by means of a heater 37 and temperature probe 38. In order to prevent Build-up of foamwithin the sludge tank 23 a fine spray of liquid is injected via valve 41 into the top of the sludge tank 23 at predetermined times. After 20 days of storage treatment there remains .an inert bulk of material in the sludge tank 23. The 20 days storage defines the maximum enduranceof theship'ssewagetreatmentplant afterwhich the inert sludge is discharged from the sludge tank via pump 39 and valve 40. In the present invention by keepingthedensityof suspended solids in the aeration tank 6 to a predeterminedπaximum, filtering of theliquid after carbonaceous oxidation/nitrification processing is optimised.

Claims: Claims 1. An aerobic sewagetreatmentplant comprising asewagecollection tank, a settlement tank, an aeration tank, transfer means and a transfer controller arranged such that sewage is transferred in controlled batches from the collection tank into the aeration tank via the settlement tank and subsequently out after separation of the sludge/effluentwherebyprocessing can be accurately controlled. 2. Asewage treatmentplant as claimed inclaim1whereinafreshwaterflush vacuum system is used to collect sewage in the collection tank. 3. A sewage treatment plant as claimed in claim1 or 2 wherein eachbatch of sewage is transferred toagreywaterdillution systemforacontrolledperiod where the sewage is dilluted tooptimisethesubsequentbiologicalprocess in the aeration tank. 4. A sewage treatment plant as claimed in claim 3 wherein the sewage is dilluted in the ratio of 15:1. 5. A sewage treatment plant as claimed in any one preceding claimwhereina sludge storage tank is provided and a suspended solids density probe is provided in the aeration tank such that sludge is automatically withdrawn fromthe aeration tank to the storage tank tomaintain the level of suspended solids within a predetermined limit. 6. Asewage treatmentplant as claimed inclaim5wherein thesludgestorage tank is providedwith aerators and its temperature is controlled such that aerobic digestion renders the sludge inert. 7. A sewage treatment plant as claimed in claim 5 or 6 wherein the sludge storage tank isprovidedwith aliquid spray, operationofwhich inhibitsthe build up of foam .above the sludge. 8. A sewage treatment plant as claimed in any one of claims5 to7 whereina dischargemeans is connected tothe sludge storage tankwhereby inert sludge is discharged f om the sludge storage tank. 9. A sewage treat tent plant as claimed in any one preceding claimwherein the aeration tank is provided with monitoring and controlling means to maintainliquidlevel, temperatureanddissolvedoxygencontentofthesewage within predetermined limits. 10. Asewage treatmentplant asclaimed inanyoneprecedingclaimwhereinthe settlment tank is disposed between the collection tank and the grey water dillution .system/aeration tank and a suspended solidsprobe isprovided such that sludge is automatically withdrawn from the settlement tank to the storage tank prior to aeration to maintain the level of suspended solids within predetermined limits. 11. Asewage treatmentplant asclaimed inany oneprecedingclaimwhereinthe controller and transfer means are arranged such that small batches of the liquid are drawnby vacuum fromtheaerationtank intoachlorinationtankvia a separation system at controlled intervals between successive sewagebatch transfers from the collection tank to the aeration tank via the settlement tank. 12. A sewage treatment plant as claimed in claim 11 wherein the separation system comprises at least one solid separator selected from the group of tilted plate separator, air floatation separator and centriguge.

Inventors: COOPER PAUL FREDERICK, FREEMAN DEREK JEREMY, LIDGITT PETER JOHN, COOPER, PAUL, FREDERICK, FREEMAN, DEREK, JEREMY, LIDGITT, PETER, JOHN
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1988
Main classification: B63B17
IPC codes: B63B17, B63B29, B63J4, C02F1, C02F3, C02F9