Treatment of liquid waste

An electrocatalytic process for treating liquid waste material containing at least one environment-polluting substance, comprising causing the liquid waste material to be treated to pass between and to contact at least one pair of spaced apart electrodes, across which a dc potential difference is applied, to cause the at least one environment-polluting organic substance to be at least partially decomposed into a gaseous or liquid form. The invention also relates to a treatment unit for performing the process.

This invention relates to an electrocatalytic process for treating liquid
 waste or effluent. In particular, but not exclusively, the invention can
 be used to breakdown long chain fat and related type molecules into basic
 gaseous elements, hence reducing the fats, oils, greases and similar
 wastes content in effluent discharges which can cause serious blockages in
 discharge pipes and problems in sewerage treatment plants.
 The use of electrolysis is known as a means to help separate fats and oils
 from liquid waste. U.S. Pat. No. 5,593,598 discloses the use of ozone
 injection and alternating direct current to convert oil and grease
 contaminated cleaning solutions back to useful polar water soluble
 surfactants, emulsifiers, etc.
 EP-A-0323690 discloses the use of proton addition by magnetic fluxing to
 help separate oils and waste water. WO-A-86/07586 discloses the use of an
 electrolytic field to help the formation of oily contaminants into a
 "floc" for easier removal. U.S. Pat. No. 5,531,865 discloses the use of an
 electrolytic field to oxidise partially fats and greases and to increase
 their absorbability with the use of flocculating agents. GB-A-1520299
 discloses the use of electrodes surrounding a closely spaced flow channel
 in which foaming and flocculation occurs to collect fats.
 All these known disclosures describe the use of electrical forces to aid
 separation of the fats, greases and oils into a floc or the like which can
 be more easily removed. However, physical removal is still required, e.g.
 by skimming, sedimentation, filtration or other means, and the resultant
 waste fats and the like which are collected still need to be re-processed
 or disposed of.
 Fats, oils and greases can be separated from liquid waste by more
 traditional methods such as dissolved air flotation (DAF); aerobic
 digestion (AD); and the use of grease traps with or without biological
 additions. DAF involves high energy costs and still requires the physical
 separation and disposal of fats. The AD process variant is a common system
 for treating a range of organic wastes and involves the use of large
 cumbersome plant. With high fat concentration in the waste, the air
 injection needs to be greatly increased resulting in large energy costs.
 Grease traps are merely physical collectors, often subject to
 maloperation, and require physical removal and disposal elsewhere of
 collected fatty waste.
 According to one aspect of the present invention there is provided an
 electrocatalytic process for treating liquid waste material.
 According to another aspect of the present invention there is provide a
 treatment unit for performing the process according to said one aspect of
 the present invention.

FIG. 1a shows a treatment unit for electrocatalytically treating liquid
 waste material containing at least environment-polluting substance,
 especially organic substances such as fatty acids, fats, oils and greases.
 The liquid waste material enters the treatment unit via a conduit 1a in
 the form of a pipe, duct or trough and leaves via a similar conduit 1b.
 The liquid waste may, if required, pass through a cleanable coarse filter
 2 before entering the treatment unit.
 The treatment unit suitably has a square or rectangular cross-section and
 has expansion joining ducts 3 at the entry and exit designed and shaped to
 join the round or square ends of the conduits 1a and 1b to the square or
 rectangular treatment unit entry and exit cross-sections. The amount of
 expansion will be calculated in each case to suit the optimum sizing for
 the treatment unit.
 Within the treatment section of the treatment unit a plurality of electrode
 plates 6 are mounted on a frame which provides conductors for the
 electrical connection of the plates to an electrical control system via
 shell connectors 5.
 The system of electrode plates 6 may comprise one electrode plate set (or
 pair) 4, or a number of electrode plate sets placed adjacent to each
 other. The plates are mounted parallel with each other. Preferably the
 plates 6 are arranged horizontally or at an angle to the horizontal but
 may instead be placed at least substantially vertically. In the preferred
 non-vertical arrangement, the top plate is the cathode and is typically
 manufactured from steel, e.g. mild steel or stainless steel, bar plated
 with nickel, cadmium or titanium (or combinations thereof) and
 subsequently machined to provide a number of ferro-nickel, ferro-cadmium
 and/or ferro-titanium interfaces on the surface. The lower plate of each
 pair is the anode and is made from uncoated stainless steel bar stock.
 Individual parts may be insulated from each other or alternate plates
 placed sequentially.
 A typical method of manufacturing a nickel coated cathode is to take a
 sheet or bar of ordinary mild steel as the substrate and to create on its
 surface a series of irregularities, in the form of trough regions and
 raised regions, by etching the steel in a bath of concentrated (50-55%)
 sulphuric acid. The natural impurity of most commonly available mild steel
 ensures that etching will take place in a random and irregular manner.
 Mostly, this is caused by the presence of finely divided granular
 alpha-ferrite which appears to be preferentially attacked by the acid.
 After inspection of the surface and the determination of the average size
 of the nodes or raised regions on the roughened steel (optimally these
 should be at 0.03-0.05 mm distribution), the surface is passivated in
 concentrated nitric acid and further passivated in a chromic acid bath.
 The roughened surface of the steel substrate is then given a 25-micron
 coating of nickel by the "electroless" process, also known as
 auto-catalytic chemical deposition. This plating process provides
 accretion of deposited nickel in the trough regions and thinner deposits
 of nickel on the raised regions. After coating, the electrode is machined
 or ground, e.g. using a finishing sander and 120 grit silicon carbide
 paper belt, to remove the "peaks" of the plated raised regions and in
 particular to remove the plated nickel from these "peaks" so as to expose
 the steel of the substrate. In this way a plurality of metal-to-metal
 interfaces are created on the active surface of the cathode between the
 nickel plated regions on the trough regions of the substrate and the
 exposed steel surfaces of the substrate. Constant microscopic inspection
 is required to determine the existence of the correct bi-metallic
 interfaces on the active surface of the electrode. Finally the electrode
 is cleaned in methyl ethyl ketone to remove grease and other machining
 deposits. This method of manufacture is described in more detail in
 GB-A-2321646.
 An alternative method of manufacturing a cathode involves preparing a sheet
 of mild steel for plating by polishing the steel plate to a 600 grit
 standard. Prior to plating all sharp edges are rounded, typically to a 1
 cm radius or less, e.g. about 1 mm, for those specific edges adjacent the
 active cathode face. A layer of nickel, typically having a thickness of
 from 50-100 micron, is then provided over the entire plate, preferably by
 an electroless nickel coating process. After this, the active cathode face
 is skimmed or scored to provide random "valleys" or recesses on the nickel
 coated steel to produce nickel/steel interfaces over a proportion, e.g.
 about 25%, of the clad or plated surface. The reverse side of the cathode,
 i.e. opposite the active cathode face, if immersed in the effluent, may be
 coated with a covering, e.g. a low temperature setting silicone-based
 rubberised covering or the like, to ensure that the surface is inert
 relative to adjacent edges of the plate.
 The clearance between the anode and cathode of each plate set will
 typically be 50-100 mm. although larger or smaller clearances may also be
 used dependent on waste stream conditions and treatment requirements.
 Typically the voltage between plates will vary from 5 to 100 volts DC.
 The sizing of the treatment unit and the plate arrangement depends on the
 concentration of fats and oils in the waste stream and the volume flow
 rate. In use, the electrocatalytic process causes proton-tunnelling in the
 cathode's ferro-nickel lattice structure. This shifts the ion products of
 the liquid waste electrolyte resulting in the fat or other pollutant, e.g.
 organic pollutant, present in the liquid waste donating a proton to the
 lattice, this destabilising the end of the fat chain and releasing
 hydrogen gas.
 The treatment process is activated adjacent to the cathode and the function
 of the anode is merely to provide a return path for current. It is
 necessary therefore to ensure the liquid waste is made to flow close to
 the cathode for sufficient time to allow the above reaction to proceed for
 the bulk of the liquid waste passing through the treatment unit. The
 length of the liquid flow path, the number of plate pairs and the flow
 velocity over them will be adjusted to achieve this objective.
 Where long electrode length is necessary it is likely that the electrodes
 will be sectioned (or split) to form a number of elements in service (see
 item 7). This may be necessary to provide voltage and current control. In
 addition such an arrangement provides spaces for upward rising released
 gases to pass to a gas discharge off-take 8 at the top of the treatment
 unit for subsequent collection, venting or burning off.
 On some installations it may be necessary to insert flow disturbers 9 at
 the entry to the electrode channels to ensure turbulence of the stream and
 improve contact with the cathode.
 On large units access doors 10, e.g. sealed, removable plates, may be
 provided on the inlet and/or outlet duct 3 while on smaller units the
 whole assembly may be readily demountable with quick fit flanges.
 In some arrangements a multi-pass liquid flow path across arrays 11 of
 electrode pairs may be provided as schematically shown in FIG. 1b.
 An alternative arrangement is shown in FIG. 2 where a treatment unit
 receives, via an inlet pipe 21, effluent containing fatty wastes. In some
 cases, such as wastes from fast food chain outlets, hotels, hospitals,
 etc., the effluent will arrive in spasmodic batches as the waste is
 produced. Prior to discharge, the waste passes through a high speed,
 in-line stirrer which, together with the addition of a controlled, small
 amount of detergent, ensures adequate emulsification of the effluent. The
 waste is received in a storage tank 22 which contains a reservoir of
 liquid. The tank 22 feeds a flow treatment unit 35. If necessary an
 additional stirrer 33 is provided in the tank 22 in order to maintain fats
 in solution as far as possible.
 Waste water is discharged via entry 24 by gravity head into the treatment
 unit 35 and is directed into an electrode plate system 25 comprising one
 or more electrode plate pairs of the same design as described previously.
 The electrodes plates may be horizontally aligned or angled (as shown) in
 order to aid flow of released gases which finally vent through a chimney
 unit 30. Effluent flow is constrained to flow back across the top of the
 plate assembly and will pass back down to the electrode plate entry
 chamber 26 via a simple one way valve 27. Hence a degree of re-circulation
 is provided to increase contact time between the waste stream and the
 cathode faces.
 If additional recirculation is required, a pumped offtake from the plate
 assembly area outlet is provided to feed either into the storage tank or
 entry pipeline upstream of the emulsifier unit.
 A steady off-take of cleaned up liquid effluent exits from the treatment
 unit 35 via outlet 28 which is sized to match inlet flow via entry 24 and
 is provided with adjustable means to ensure that the necessary flow
 balancing is achieved.
 A lower outlet 29 is provided for the treatment unit which can be opened
 manually or by remote initiation to enable the system to be emptied as and
 when required.
 The overall system has to be carefully sized (both in terms of reservoir
 size, treatment module, and controlled flow rates) in relation to the
 nature and flow pattern of the waste stream being treated.
 FIG. 3 shows an anaerobic chamber comprising a circular tank 41 provided
 with a domed top in which a gas collection pipe 42 is located. A stirrer
 43 is fitted to mix the contents. Biodegradable liquid effluent waste
 enters at inlet 44 and remains in the tank for some days during the normal
 digestion process. In this period the liquid effluent waste circulates
 within the chamber both due to thermal effects and due to the action of
 the stirrer 43. Treated liquid effluent exits from the chamber via a top
 outlet 48.
 Within the chamber there is located a cathode 45 of the same construction
 as described previously and which is arranged horizontally just below the
 waste liquid level, and a corresponding anode 46 which is located inset in
 the base of the chamber. Alternatively a different form of anode 47 may be
 located closely below the cathode 45 if the distance to the base is too
 great to create the necessary current density. Such an anode 47 is
 suitably of an open mesh type structure to allow wastes to flow past it.
 With this system the electrolytic digester breaks down the long chain fats
 which will normally retard the main anaerobic digestion process, and
 therefore allow the AD process to proceed efficiently.
 Although the spaced apart electrodes of each electrode pair in the
 embodiments described are preferably arranged parallel to each other and
 are flat or plate-like, other forms of spaced apart electrodes, e.g.
 coaxially arranged electrodes, may be provided.