Anaerobic treatment of wastes containing insoluble components

Details Anaerobic treatment of wastes containing insoluble components Anaerobic treatment of wastes containing insoluble components

2003-02-03

2004-11-30

Prince, Fred G. (Department: 1724)

Liquid purification or separation

Processes

Treatment by living organism

C210S612000, C210S631000, C210S908000, C435S262500

Reexamination Certificate

active

06824683

ABSTRACT:

This invention relates to a waste treatment process and plant which is suitable for treatment of waste inclusive of biological waste exemplified by faeces, sewage and household waste and commercial and industrial waste inclusive of greasetrap waste.
Reference may be made to International Patent Publication WO 95/25071 which refers to waste treatment plant and process which includes the steps of:
(i) passing waste material comprising an insoluble component through a bioreactor system including a plurality of bioreactors in series and maintaining said insoluble component as a suspension in said waste material;
(ii) passing treated waste material from said bioreactor is system to one or more acidification tanks to reduce the pH below 4.5 to produce free volatile fatty acids for elimination of bacterial pathogens in said treated waste material; and
(iii) separating the insoluble component from the waste material before or after step (ii).
It was an essential feature of the invention disclosed in International Publication WO 95/25071 that after treatment of the waste material in the bioreactor system that the treated waste material was passed to one or more acidification tanks to facilitate the elimination of bacterial pathogens by free volatile fatty acids (VFAs).
However, unexpectedly, it has now been found that it is more efficient, in relation to some waste, inclusive of greasetrap waste, in disposal or elimination of bacterial pathogens, to maintain the pH of the waste material below 4.9 for at least 24 hours when the waste material is being passed through the bioreactor system. This means that an effective process of waste treatment may take place which can eliminate step (ii) of the process of WO 95/25071 to thereby produce a more simplified and economical process of waste treatment.
Therefore the invention provides a process of waste treatment which includes the following steps:—
(a) passing waste material comprising an insoluble component through an anaerobic bioreactor system optionally including a plurality of bioreactors in series and maintaining said insoluble component as a suspension in said waste material, whereby the pH of the waste material is maintained below 4.9 for a period of at least 24 hours;
(b) aerating the waste material after step (a); and
(c) removal of the insoluble component from the waste material before or after step (b).
The waste material will usually, in most circumstances, have a pH of around 5.3-5.8 and thus will need to be subjected to a pH lowering step prior to being fed into the bioreactor system or when passing through the anaerobic bioreactor system. More preferably, the pH lowering step will take place when the waste material is passing through the bioreactor system. However, it will be appreciated that in some circumstances, because of the nature of the waste material, prior to being fed into the bioreactor system, a pH lowering step may not be necessary. However, this event is relatively rare. In any event it will be appreciated that the pH of the waste material will be monitored as it passes through the anaerobic bioreactor system.
Usually the pH lowering step will involve the addition of a strong mineral acid such as hydrochloric acid, sulphuric acid or nitric acid. However, this does not preclude the use of other acids such as phosphoric acid, perchloric acid or strong organic acids which will achieve the same effect.
The bioreactor system will comprise a plurality of bioreactors as described in WO 95/25071 the contents of which are totally incorporated herein by reference.
However, as in the case of WO 95/25071, each bioreactor may be interconnected by an overflow conduit so that waste material or effluent is quickly and efficiently transferred from one bioreactor to an adjacent bioreactor without the need for pumping material so as to transfer material from one bioreactor to another. Suitably each bioreactor is provided with agitation means which keeps the contents of each bioreactor in the form of a slurry or suspension so that solid particles are maintained in a suspended state.
The contents of each bioreactor may be also subject to heating means and in one form this may be provided by steam being passed into and out of each bioreactor. However, other forms of heating means may be adopted, such as electrical heating. Preferably, the temperature in each bioreactor is maintained by suitably thermostatically controlled means between 25-40° C. and more suitably 30-40° C.
Preferably, the pH of a first bioreactor is maintained between 4.8-5.5 and more preferably around 5.0. The pH of the second bioreactor is suitably maintained between 3.6-4.9, more preferably between 4.0-4.7 and most preferably, about 4.3 Preferably, the retention time in each bioreactor may be 12-48 hours, but more suitably is 24 hours.
The waste material which is subject to the process of the invention may comprise human or animal faeces and preferably faeces from livestock feedlots which may have a stockfeed component containing lignocellulose. In fact the process of the invention is extremely efficient in treatment of waste which requires an initial anaerobic fermentation step to break down complex macromolecules such as carbohydrates and proteins to short chain organic acids of 8 carbon atoms or less. Also complex macromolecules such as lipids may be broken down to long chain organic acids of 9 carbon atoms or more and glycerol. This fermentation step usually takes place in the presence of acidogenic fermentative bacteria and lipolytic bacteria which can produce organic acids such as volatile fatty acids and long chain fatty acids, which may be readily metabolised to carbon dioxide and water.
In the initial anaerobic fermentation step use may be made of a series of anaerobic bioreactors as described above which are designed so that oxygen or air is prevented from being introduced into each of the bioreactors. Usually the amount of dissolved oxygen will be very low and be less than 0.7 mg/l. Such bioreactors therefore may be sealed from atmosphere.
After passage through a number of anaerobic reactors e.g. usually from 2-6 and more preferably from 3-6 and suitably 5 in number, the waste so treated may then be passed to an aerobic tank or aerated system. It may comprise one cell or a plurality of aerobic or aerated cells wherein an air line may supply air or oxygen to aerators or jets which feed the air under pressure into the cell(s). Usually the aerators may function under diffusion but mechanical aerators may also be utilised. The aerator may be of plug-flow configuration as described in U.S. Pat. No. 5,380,438 or may be of a step-feed and complete-mix aeration configuration or alternatively an activated sludge system.
Most preferably the aeration system may comprise a tank having a sparger at the bottom of the tank, in which air is fed into the tank under pressure.
If desired, the waste material after passage from the anaerobic bioreactor system may be subjected to a separation procedure to remove a solid component such as sludge. This may be achieved by passing the waste material over a screen which is subject to the action of wash water above and below the screen to prevent clogging or blockages occurring in pores or passages located in the screen. Preferably use may be made of a filter system known as the BAYLEEN filter system.
Reference may now be made to a preferred embodiment of the invention as shown in the attached drawing in FIG. 1, wherein initially, faeces from an animal feedlot or commercial or industrial waste such as greasetrap waste are delivered to a loading bay
10
wherein waste may be conveyed to holding tanks
14
A,
14
B and
14
C along conduits
11
,
12
and
13
respectively. If desired low strength organic waste having a low BOD of around 2000-3000 mg/l may be delivered to a composting area through conduit
9
.
Initially the waste may be passed through a hydrocyclone or centrifugal separator (not shown) to remove large foreign bodies such as spoons, knives, nuts, bolts etc. Loading bay
10
may also comprise a macerator pump for transp

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