Abstract:
The apparatus comprises an anaerobic digester for receiving organic waste, an aerobic digester, and a feed pump for pumping effluent from the anaerobic digester to the aerobic digester. The digesters each comprise a reaction vessel and each reaction vessel has a spray nozzle at or adjacent to its apex for spraying an anti-foam liquid at the contents of the vessel. A method of treating organic waste is also disclosed.

Description:
This invention relates to apparatus for and a method of treating organic waste such as sewage sludge and animal manures. 
     The enforcement of EC Directives and USEPA guidelines on disposal of organic waste and their use in agriculture has brought about changes in sludge disposal practice as well as a need to study the options available and their cost-effectiveness. Treatment is required to reduce significantly the Pollution content of the sludge as well as its health hazard due to the presence of pathogens. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided apparatus for treating organic waste, comprising an anaerobic digester for receiving the organic waste, an aerobic digester, and means for pumping effluent from the anaerobic digester to the aerobic digester, the digesters each having a reaction vessel and each reaction vessel having a spray nozzle at or adjacent to its upper end for spraying an anti-foam liquid onto the contents of the vessel. 
     The anti-foam liquid could be water or a mixture of water and an anti-foam agent. 
     According to a second aspect of the invention there is provided a method of the steps of:
         a. feeding the organic waste into an anaerobic digester.   b. feeding and mixing the waste in the digester contents in a predetermined controlled cycle.   c. pumping effluent from the anaerobic digester to an aerobic digester.   d. mixing the organic waste in the aerobic digester with air by pumping the organic waste through a Venturi mixer which draws air into the organic effluent.   e. measuring the organic content of the effluent fed into the aerobic digester, and   f. varying the flow rate at which organic waste is pumped through the Venturi mixer according to the volume and organic content of the sludge fed into the aerobic digester, and   g. spraying an anti-foam liquid at the contents of both of the digesters.       

     The invention will now be more particularly described, by way of example, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of one embodiment of apparatus according to the first aspect of the invention, 
         FIG. 2  is an enlarged view of the spray nozzle fitted to each reaction vessel of the apparatus, and 
         FIG. 3  shows a distribution network for the spray nozzles. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring firstly to  FIG. 1  of the drawings, the apparatus shown therein comprises an anaerobic digester  10 , an aerobic digester  11 , a feed line  12  connected between the upper end of the anaerobic digester  10  and the lower end of the aerobic digester  11  and a feed pump  13  for pumping effluent from the anaerobic digester  10  to the aerobic digester  11  through the line  12 . 
     The anaerobic digester  10  comprises an insulated anaerobic reaction vessel  14  having an inlet pipe  15 , a recycling pipe  16  and a recycle pump  17  for taking gas produced in the vessel  14  from the roof of the vessel  14  to the nozzle  18  in the centre of the base of the vessel  14 . The recycle pump  17  is switched on intermittently and only after the feeding regime is concluded on a regular cycle. This can be hourly or whatever regular cycle chosen for a particular application. Feeding and mixing organic waste in the digester contents is thus in a predetermined controlled cycle. This produces a gentle rolling action for mixing the contents of the vessel. 
     The anaerobic digester  10  also includes a settlement tube  19  against the inside wall of the vessel  14 . This has an inlet  20  above the base of the vessel and increases in cross-sectional dimensions as it extends upwards to the level at which effluent exits the vessel  14 . The settlement tube allows the digester contents to exit the vessel  14 . The increasing cross-sectional area as the contents move upwards reduces the rate of upflow speed of the contained particles. As a result the larger particles will tend to slow down as well as fall back and remain in the tank for longer than they would normally. This has the overall result of increasing the solids retention time. 
     Typically the organic waste will remain in the anaerobic digester for between about 6 and 15 days depending on the strength of the waste. Taking, by way of example, a vessel  14  having a capacity of 1,000 cubic metres and a retention time in the digester of 10 days, 100 cubic metres will be fed into the digester  10  each day. The feed pump (not shown) should be activated for predetermined periods related to the hydraulic retention time. As organic waste is fed into the anaerobic digester  10 , effluent will leave the digester  10  via the feed line  12 . This effluent can be fed directly to the aerobic digester  11  as shown in  FIG. 1  or can be fed into a holding tank or reservoir (not shown) for subsequent transfer to the aerobic digester  11 . Once the feeding regime to the digester  10  has finished, the pump  17  is switched on for a period required to provide adequate mixing in the reaction vessel  14 . Typically this could be for a duration of from 5 to 30 minutes and is determined by the solid concentration of the material to be mixed. The higher the total solids concentration, the longer the material is mixed. After a further delay, typically of between 1 and 6 hours the cycle begins again with the feeding regime outlined above. 
     The aerobic digester  11  comprises an insulated aerobic reaction vessel  22 , a Venturi mixer  23 , a liquid outlet  24  and an air outlet  25 . The Venturi mixer  23  comprises a recycle tube  26  connecting the lower end of the vessel  22  to the upper end of the vessel  22 , an air inlet tube  27  connected to the recycle tube  26  intermediate the ends thereof and a recycle pump  28  for recycling effluent from the anaerobic digester  10  through the recycle tube  26  and past the inner end of the air inlet tube  27 . 
     The line  12  is connected to the recycle tube  26  at the lower end thereof and upstream of the recycle pump  28 . 
     Two temperature sensors  29 ,  30  are mounted in the reaction vessel  22 . The temperature sensor  29  is mounted at a position above the sensor  30  so that the temperature of the effluent at two levels in the reaction vessel  22  can be monitored. 
     A syphon break tube  31  is connected to the liquid outlet  24 . 
     In operation, effluent from the digester  10  is fed into the aerobic reaction vessel  22  through the inlet line  12  by the feed pump  13  and is recycled in the aerobic reaction vessel  10  through the Venturi mixer  23  by the recycle pump  28 . 
     As the effluent is pumped through the Venturi mixer  23  by the pump  28 , air is drawn into the effluent from the air inlet tube  27 . The quantity of air drawn into the reaction vessel  22  in a given period of time can be varied by varying the flow rate at which the effluent is pumped through the Venturi mixer  23 . 
     Because the effluent to be treated may vary as to its organic content (as measured by its contained volatile solids or Biological Oxygen Demand), it becomes important to control the rate of oxidation by means of knowing the incoming organic loading of the incoming effluent. 
     As the organic loading (measured in terms of kg of Volatile Solids per M 3  of reactor per day for example) increases, the rate of oxidation and mixing can be increased to provide sufficient aeration capacity for the microbes providing the metabolic heat. This process can be effected automatically by means of an automated BOD sensor and used to control the flow rate at which the effluent is pumped through the Venturi mixer  23 . Thus, the quantity of air drawn into the effluent can be matched to the volume and strength of effluent fed to the reaction vessel  22  from knowledge of the volume of effluent fed into the vessel  22  by the feed pump  13  and from the organic content of that effluent. 
     In order to ensure that thermophylic aerobic digestion takes place, the effluent in the aerobic reaction vessel  22  must be maintained above a predetermined temperature and is usually maintained at between 55° C. and 70° C. The temperature sensors  29  and  30  monitor the temperature of effluent in the reaction vessel  22  and if this temperature falls below a predetermined value, the quantity of effluent fed into the reaction vessel  22  over a given time period is increased to increase the oxidisable organic carbon in the reaction vessel  22  and/or the flow rate at which the effluent is pumped through the venturi mixer  23  is increased to increase the quantity of air in the reaction vessel  22 . The quantity of effluent fed into the reaction vessel  22  can be increased either by increasing the frequency at which the feed pump  13  is operated or by increasing the duration of feed. 
     In practice, the feed pump  13  will be operated intermittently, e.g. for about 10 minutes every three hours, and during operation of the feed pump  13 , the recycle pump  28  can be switched off. This enables the treated effluent to be discharged from the vessel  22  before new effluent is mixed by the Venturi mixer  23  with the contents of the vessel  22 . In this case, the incoming effluent will be fed into the lower end of the vessel  22 . 
     Alternatively, the recycle pump  28  can remain on while the feed pump  13  is operating. In this case, some of the effluent will be fed into the lower end of the vessel  22  passing through the lower end of the recycle tube  26  against the flow of effluent being recycled through the recycle tube  26  and some of the effluent will be carried round with the recycled effluent and will be fed into the upper end of the reaction vessel  22 . 
     The rate at which effluent is pumped through the Venturi mixer  23  can also be increased by increasing the speed of the recycle pump  28  if the differential temperature sensed by the temperature sensors  29  and  30  exceeds a certain value as this will indicate insufficient mixing of the contents of the vessel  22 . 
     Foaming problems have occurred both in anaerobic and aerobic digesters and it is often not possible to identify the single causative agent, although it is generally accepted that textile industry effluent can cause foaming problems in a number of treatment processes. An anti-foam system is therefore included in each digester  10 ,  11 . Without this system the digesters can only be operated at 50–70% of their design load and cannot treat all effluent produced on site. As best shown in  FIG. 2 , each foam system comprises a spray nozzle  35  mounted in an upstanding extension tube  36  at the apex of each reaction vessel  14 ,  22 . The extension tube  36  has a viewing window  37  at its upper end and a gas vent  38  extending laterally and upwardly from the side of the upstanding extension tube  36 . As shown in  FIG. 3 , the nozzles  35  are supplied with an anti-foam liquid, which could be water or a mixture of water and an anti-foam agent, from a tank  40  by pump  41 . An isolating valve  42  and a non-return valve  43  are provided in the common supply line  44  for the nozzles  35  and each nozzle  35  also has its own dedicated isolating valve  45 , its own dedicated solenoid valve  46 , its own dedicated drain valve  47  in the supply to the nozzle  35  and its own dedicated pressure switch  48 . The distribution pipes are protected from freezing by trace heating  49 . 
     The nozzles  35  are used to evenly distribute the anti-foam liquid to the reaction vessel to physically disrupt the foam structure within the vessel. 
     Preferably, a reverse osmosis device  50  is provided downstream of the aerobic digester  11  and/or the anaerobic digester  10 . The reverse osmosis device(s)  50  contains at least one semi-permeable membrane which allows dirty effluent from the digester to be cleaned when passing through it. The contained, dissolved and suspended solids are removed allowing the clean water to pass through the membrane under pressure and be recovered. Reverse osmosis is capable of removing bacteria, salts and dissolved organics from the anaerobic or aerobic digester effluent. The removal of charged salts with reverse osmosis is helped by the natural electrical charge on the particles. Many of the organic particles are charged as well as the inorganic molecules. Thus both are removed, but especially the latter group. 
     Modern reverse osmosis technology uses at least one cross-flow membrane to allow the membrane to continually clean itself. Reverse osmosis requires a driving force to force the fluid through the membrane and uses pressure from a pump (not shown). 
     It is possible to produce 90%–95% pure effluent with the apparatus described above and this can be recycled back to dilute the organic waste being fed into the anaerobic digester  10 . This can result in a huge saving of clean water supplies which are at a premium particularly in some parts of the world. The recycling of the clean water to the beginning of the incoming flow at the anaerobic digester inlet allows a control of the concentration of waste feed to the digester  10 . 
     A computer control system has been developed for the apparatus to enable remote monitoring and control. All pumps, motors, agitators etc. can be manually controlled locally in order to facilitate system check-out and start-up. Each device has a local auto/manual selector switch which is interlocked into the logic of the control system. For safety, placing any device in manual mode locks out the automatic mode. 
     Automatic sequencing for the anaerobic digester is accomplished through the use of a programmable logic controller (PLC) in the control room. All analogue and digital I/O passes through a PLC, which allows all devices to be monitored and controlled automatically. All fixed sequences and timing are controlled from the PLC. 
     Supervisory process control and data acquisition are accomplished through an IBM compatible computer (PC) operating under OS/2 or equivalent. The PC provides sufficient processing power to operate the control software and other programs in true multi-tasking mode. A hard disk provides local storage for programs and operating/laboratory data. The PC communicates with the PLC over an RS-232 interface using the MODBUS communications protocol or equivalent. A high speed internal modem provides remote communications capability for control and data exchange. 
     The above embodiment is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined by the appended claims.