Process for the treatment of waste

A method of treating organic waste is provided which includes separating the liquid portion of the waste from the solid portion prior to reacting said solid portion in an accelerated wet oxidation reaction. The method includes using an internally-derived ash from the wet oxidation reaction to weight the organic waste, thereby increasing the rate at which the liquid phase can be separated from the solid phase. By first removing the liquid portion of the waste, the oxygen demand of the waste to be processed by wet oxidation is substantially lowered. Ammonia is removed from the liquid portion of the waste in a de-ammoniating step which is followed by biological decomposition to form a liquid stream having a greatly reduced oxygen demand. In one embodiment, the method includes further treating the liquid stream to substantially remove salts and using the resulting deionized stream as a diluent to dilute the solid portion of the waste prior to wet oxidation. The present invention also provides a method of treating organic waste which includes reacting said waste in accelerated wet oxidation reaction apparatus to produce a low-volume sterile ash, a liquid effluent portion and a mixture of reaction off gases. The liquid effluent portion is de-ammoniated then decomposed biologically to form a liquid stream having a greatly reduced oxygen demand. The liquid stream is then desalinated and used as a diluent to dilute the organic waste prior to its introduction into the wet oxidation reaction apparatus.

FIELD OF INVENTION 
The present invention relates generally to methods for treating organic 
waste and wastewater. More specifically the present invention relates to 
integrated processes which utilize physical, biological and chemical 
operations for the treatment of animal waste. 
BACKGROUND OF THE INVENTION 
It is estimated that in the United States alone, over 1.5 billion tons of 
animal waste are produced annually. While a portion of this waste is 
generated on farms where disposal may consist of on-site manure spreading, 
large amounts are also produced in feedlots, many of which are located 
near urban areas. For a number of reasons, manure spreading, incineration 
and other traditional waste disposal alternatives may be undesirable or 
impractical. Hence, economical animal waste processing methods which 
produce environmentally desirable products are needed. 
Modern animal feedstuffs are complex mixtures of protein, fiber, vitamins, 
minerals, salts and other components, balanced to provide optimum 
nutrition at minimum cost. Sodium, calcium, phosphorous, magnesium, 
potassium, and other minerals are present in most feedstuffs. In addition, 
large amounts of chlorides are ingested by livestock, often from 
salt-blocks and the like. The ingestion of salt is known to increase the 
thirst and appetite of animals which increases body weight. Although 
nutrient-enriched diets are designed to produce healthier, more marketable 
likestock, the high concentrations of chlorides, ammonia, and minerals 
which these highly nutritious feedstuffs produce in animal waste are of 
particular concern in waste processing systems. 
Generally, livestock waste contains from about 8 percent to about 11 
percent by weight carbonaceous material, although concentrations outside 
this range are not uncommon, depending on the amount of wash-down water 
present. Unprocessed or raw animal wastes typically have a chemical oxygen 
demand (COD) of from about 80,000 mg/l to about 120,000 mg/l. The chemical 
oxygen demand is a measure of the quantity of chemically oxidizable 
components present in the waste. Animal wastes also generally have a 
biochemical oxygen demand (BOD) of from about 40,000 gm/l to about 90,000 
mg/l. The biochemical oxygen demand is the quantity of oxygen required 
during decomposition of the organic waste matter by aerobic biochemical 
action. The BOD is usually determined by measuring oxygen consumption of 
decay microorganisms in a waste sample during a five-day period at 
20.degree. C. As will be appreciated by those skilled in the art, one 
important objective of waste treatment processing is the reduction of the 
oxygen demand of the waste effluent. Oxygen depletion or deoxygenation of 
receiving waters due to the discharge of waste effluents having high 
oxygen demands is a significant environmental concern and is the subject 
of considerable governmental regulation. 
Waste processing systems are designed to provide the most environmentally 
compatible waste effluent at the lowest cost, with maximum utilization of 
process by-products. Thus, processing schemes which require a relatively 
low initial capital expenditure and which have low operational and 
maintenance costs are highly desirable. Those conventional waste 
processing systems which are adaptable to processing animal wastes lack 
efficient, comprehensive methods for substantially reducing the oxygen 
demand of the wastes while also removing harmful chlorides and the like 
which cause corrosion of the waste processing equipment. Further, 
conventional processes do not adequately inhibit the formation or transfer 
of environmentally undesirable products. The present invention provides an 
integrated waste treatment system particularly suited for the processing 
of animal wastes which solves many of the problems associated with 
conventional waste processing systems. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a total, 
integrated system for processing waste, which is particularly suitable for 
processing animal waste. In one embodiment, the method includes adding an 
internally-derived ash as a weighting agent to animal waste, then 
dewatering the weighted waste to form a liquid stream which may contain as 
much as half of the biochemical oxygen demand of the initial waste. This 
liquid stream also contains a substantial portion of the chlorides and 
ammonia present in the waste. The liquid stream is then de-ammoniated, 
preferably by ammonia stripping, and the resulting ammonia compounds are 
reclaimed for use as fertilizer or the like. The de-ammoniated liquid 
stream is flowed directly to a biological polishing unit where suspended 
solids and dissolved organic matter are decomposed efficiently. The 
preferred biological processing step is anaerobic polishing in which 
carbonaceous material is converted to methane and other gases which can 
then be used as fuel, preferably on-site. The biological sludge or biomass 
produced during biological polishing is recombined with the dewatered 
waste which undergoes thermal treatment (aerobic or anaerobic). 
Preferably, anions such as phosphate, sulfate, carbonate and oxalate ions 
are removed from the liquid stream during the de-ammoniating process. By 
adding calcium hydroxide or another suitable precipitant to the liquid 
stream in the present invention, these anions are precipitated out of 
solution. Simultaneously, the addition of calcium hydroxide raises the pH 
of the liquid stream so that ammonia can be removed in an ammonia 
stripper. The precipitates are separated from the stream and added to the 
ash derived from thermal treatment of the carbonaceous solids, and then 
dewatered for disposal or recovery. 
The present invention also contemplates the use of internally-derived 
gaseous carbon dioxide to carbonate the liquid stream following the anion 
removal/ammonia stripping step. This is achieved in one aspect by bubbling 
biogas generated during biological polishing or during a subsequent wet 
oxidation stage through the de-ammoniated liquid stream to precipitate 
scale-forming cations such as calcium and magnesium which may otherwise 
form scale on the reaction apparatus during subsequent waste treatment. 
The resulting carbonate precipitates are then added to the ash to be 
dewatered and undergo disposal as described herein. 
The liquid effluent stream, after biological polishing, is then preferably 
further treated by aerobic polishing, evaporation/condensation or most 
preferably by ultrafiltration and reverse osmosis. The substantially 
deionized liquid stream is then preferably used as a feed diluent for 
diluting the dewatered waste which also contains the aforementioned ash 
and biomass. Alternatively, externally-derived, low-chloride water may be 
used as the feed diluent. The diluted liquid waste is then flowed into a 
wet oxidation reaction apparatus where the waste is heated and mixed with 
oxygen as it is substantially, completely oxidized in an accelerated wet 
oxidation reaction. The exothermic wet oxidation reaction takes place at 
about 420.degree.-600.degree. F., preferably between 530.degree. and 
550.degree. F. The heat generated by the reaction may be recovered as 
steam of 150-1000 psig, preferably 300-600 psig, to be used along with the 
aforementioned internally-derived methane to generate electricity, 
superheated steam or hot water. The influent liquid may also be preheated 
as may be required. The effluent from the wet oxidation reaction is then 
processed in gas/liquid and liquid/solids separating equipment which 
preferably includes the steps of clarification, thickening and dewatering. 
A portion of the resulting low-volume, sterile ash is used as the 
weighting agent which is combined with the waste during the initial waste 
dewatering process. The remaining ash may be reused as an animal food 
supplement or soil conditioner. The liquid stream from the wet oxidation 
reaction effluent is flowed to the biological polishing unit to be 
processed along with the de-ammoniated liquid stream. 
By first separating the soluble COD from the waste before the dewatered 
waste is oxidized through wet oxidation, a greater concentration of waste 
can be processed in the wet oxidation apparatus. Further, removing the 
highly ammoniated liquid from the waste before the waste is oxidized 
through wet oxidation facilitates removal of ammonia to levels that do not 
inhibit biological polishing and reduces the production of undesirable 
nitrates. By using the internally-driven ash from the wet oxidation 
reaction as a weighting agent which is added to the waste for the initial 
dewatering, the sedimentation rate of the waste is enhanced considerably. 
Also, since the ash derived from the wet oxidation reaction generally 
retains some COD, by using it as a weighting agent which then passes once 
again through the wet oxidation reaction apparatus, this remaining COD is 
further reduced. 
In another embodiment, live stock or other organic waste is diluted and fed 
directly into a wet oxidation reaction apparatus along with a source of 
oxygen. At an elevated temperature and under pressure, the waste is 
oxidized through a wet oxidation reaction whereby the chemical oxygen 
demand of the waste is substantially reduced. The reaction products or 
effluent are then dewatered and the liquid portion so produced is 
processed by the method previously described which includes de-ammoniating 
the liquid portion, precipitating out scale-forming ions, polishing the 
liquid portion in a biological unit and, optionally, treating the liquid 
portion by ultrafiltration and reverse osmosis. This treatment 
substantially eliminates any remaining COD. The substantially deionized 
liquid stream is then preferably used as a feed diluent for the waste 
which is flowed into the wet oxidation reaction apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, the waste 20 to be processed, preferably animal 
waste such as swine manure is combined with an internally-derived ash 22 
at mixer 23, and the mixture 21 is dewatered by liquid/solids separation 
26. The ash acts as a weighting agent or body feed to enhance the 
sedimentation of the raw waste during liquid/solids separation. Other 
benefits of the present invention may be realized without the addition of 
a weighting agent although it is preferred that a weighting agent be used. 
Raw waste does not settle rapidly. By combining it with ash, a heavier 
mass is created which settles more quickly, thus facilitating separation. 
It is believed particles of ash serve as nuclei for the formation of 
flocculent solids which then settle more rapidly. By using an 
internally-derived ash, no additional solids are added to the system. Ash 
22 is preferably mixed with waste 20 to evenly distribute it in waste 20. 
Other advantages of using this internally-derived ash 22 as a weighting 
agent will be described more fully hereinafter. The weighted waste mixture 
21 is suitably dewatered by centrifugation, vacuum filtration, belt 
filtration or simply by allowing the weighted waste to settle in response 
to gravity. Other waste dewatering techniques including the use of a 
coagulant aid may be suitable or desirable in a particular application. 
Dewatering produces liquid stream 28 and dewatered waste 27. Liquid stream 
28 may contain up to 50 percent of the chemical oxygen demand of the 
initial waste 20. Typical animal waste may have a COD of from about 80,000 
mg/l to about 120,000 mg/l. Liquid stream 28 contains the dissolved COD 
which may be from about 30,000 mg/l to about 60,000 mg/l. In addition, 
liquid stream 28 may contain most of the phosphorous compounds (70-400 
mg/l) which were present in the initial waste, from bout 3,000 to about 
6,000 mg/l of ammonia compounds and considerable quantities of soluble 
salts including chlorides. Liquid stream 28 is then de-ammoniated, 
preferably by an ammonia-stripping procedure 30 which may include raising 
the pH of the liquid stream to drive off the ammonia in the form of a 
vapor. This vapor can then be condensed as ammonium hydroxide which is 
preferably reclaimed for use as a fertilizer 32. It may be possible to 
de-ammoniate liquid stream 28 using a biological reactor; however, 
conventional microbes will generally not tolerate the high concentration 
of ammonia present in the liquid stream and, thus, substantial dilution of 
liquid stream 28 would be required. The ammonia vapors can be readily 
collected using a scrubbing stream, although other methods may be 
acceptable. While substantially all of the ammonia is removed from liquid 
stream 28, a sufficient quantity is left to act as a nutrient for 
subsequent biological processing. 
It is most preferred that precipitate-forming anions such as phosphate, 
sulfate, carbonate and oxalate ions be removed from liquid stream 28 
during the de-ammoniating process. This is achieved by adding calcium 
hydroxide or another suitable precipitant to the liquid stream 28 to raise 
the pH, thereby vaporizing the ammonia, and to precipitate these ions out 
of solution. The precipitates 31 are then directed to ash liquid/solids 
separator 64 where they are dewatered with the weighted ash. Following 
ammonia stripping/anion removal, liquid stream 34 is preferably treated to 
remove potentially scale-forming cations such as a calcium and magnesium 
ions. By precipitating these cations with a precipitant which forms 
relatively insoluble precipitates, the potential to form scale during 
subsequent wet oxidation is eliminated. In the present invention, these 
cations are removed by combining liquid stream 34 with carbon dioxide in 
precipitator 35 to form carbonate precipitates 135 which are then directed 
to ash liquid/solids separator 64 to be dewatered. The CO.sub.2 used in 
this step is preferably derived as biogas 38 from biological polishing 
unit 36 during a subsequent step. 
The de-ammoniated liquid stream 34, which is substantially deionized and 
which contains enough phosphorous to meet biological nutrient 
requirements, is then flowed to a biological polishing unit 36 where 
suspended and dissolved carbonaceous matter, remaining sulfates and 
nitrates are substantially decomposed by microbes, preferably anaerobic 
microorganisms such as anaerobic bacteria. Although aerobic microbes could 
be used, this alternative would require that oxygen be supplied to 
biological polishing unit 36 and, more importantly, that liquid stream 34 
be substantially diluted. This is due to the fact that aerobic microbes 
cannot tolerate high COD concentrations at reasonable hydraulic retention 
times. During anaerobic polishing, biogas 38 is evolved, principally 
comprising methane and carbon dioxide. Smaller amounts of carbon monoxide 
and hydrogen sulfide may also be generated. Biogas 38 is preferably 
recovered, and in some applications it may be practical to separate the 
constituent gases. Biogas 38 is preferably used on-site to generate heat. 
While liquid stream 34 may have a COD as high as 60,000 mg/l, after 
processing in anaerobic polishing unit 36, the resulting liquid stream 40 
has a COD of about 1,000 to 2,000 mg/l. There is also produced a biomass 
or biological sludge 39 during biological polishing consisting of microbes 
which may represent from about 0.1 to about 2.0 percent by weight of the 
COD of liquid stream 34 where the polishing is carried out by anaerobic 
microbes. The percentage of biomass 39 is much greater when aerobic 
microbes are used in aerobic polishing. Biomass 39 is recombined with the 
initial dewatered waste 27 which undergoes further processing as will be 
explained herein. 
Depending upon how stringent the effluent criteria are in a particular 
setting, liquid stream 40 may then be treated by aerobic polishing (not 
shown), provided the prior biological polishing step consisted of 
anaerobic polishing. This further reduces the COD in many instances to as 
low as 20 to 30 mg/l. Alternatively, and preferably for use in the present 
invention, liquid stream 40 receives further total solids removal. The 
preferred filtration process includes ultrafiltration 42, followed by 
reverse osmosis unit 44. If necessary, coagulation and media filtration 
(not shown) may precede ultrafiltration. Both techniques are membrane 
filtration processes. Ultrafiltration is a "large screen" process which 
removes particulate matter having a diameter greater than about 0.45 
micrometer. After the particulate matter has been removed by filtration 
unit 42, liquid stream 46 is flowed to a reverse osmosis unit 44 where 
substantially all of the dissolved salts, including chlorides, are 
removed. After ultrafiltration 42 and reverse osmosis 44, liquid stream 48 
is substantially pure, deionized water. In a modification of the present 
invention shown in FIG. 2, ultrafiltration 42 and reverse osmosis 
treatment 44 are replaced by an evaporation step 50 in which water is 
driven off to be condensed into a fluild stream 52, leaving any salts and 
very fine particulate matter behind in the form of a brine or cake. 
Evaporation 50 is not necessarily preferred for use herein, however, 
because volatile organics may be evaporated with the water during 
condensation, causing contamination. It is important to note that either 
ultrafiltration 42 and reverse osmosis 44, or evaporation 50 produces a 
liquid stream which is substantially free of chlorides, magnesium, 
calcium, and other such substances which are generally considered 
corrosive when brought in contact with metal surfaces. The deionized or 
demineralized liquid stream 48 is then discharged or reused, preferably as 
a feed diluent to dilute the initial dewatered waste 27 at mixer 54 to 
form diluted waste 56. An external source of low-chloride water could be 
used as the feed diluent. 
If total desalination of the discharge stream is not required, split stream 
treatment may be employed to meet the desired effluent dissolved solids 
content. In split stream treatment, only part of the biological polishing 
unit effluent 40 is desalinated by membrane filtration or evaporation. The 
desalinated stream is then recombined with that portion of the stream 
which is not desalinated by membrane or evaporation resulting in a net 
reduction of dissolved solids. 
The diluted waste 56 is then preferably treated by wet oxidation in wet 
oxidation reaction apparatus 58. It is to be understood that one of the 
important advantages of the present invention is the substantial reduction 
of the oxygen demand of the waste by first separating and then diverting 
the liquid portion containing the dissolved oxygen demand through the 
aforementioned treatment steps. By removing the dissolved COD from the 
waste before it is subjected to an accelerated wet oxidation reaction, 
several benefits are achieved. A greater concentration of wastes can be 
flowed into the oxidation unit since the waste has a substantially reduced 
COD. This allows for better utilization of the wet oxidation reaction 
apparatus to convert the solid COD to liquid which is more rapidly 
digested by the biological polishing unit. Also, by first separating the 
liquid stream from the waste, corrosion of the wet oxidation reaction 
apparatus is substantially reduced by virtue of the reduced chloride 
content. 
Particularly with animal waste, during wet oxidation, substantial corrosion 
of even high grade stainless steel reaction vessels may occur due to the 
high concentration of chlorides in the waste. Chlorides, sulfates, 
phosphate, carbonates as well as calcium and other ions which are 
substantially removed by the method of the present invention, form 
insoluble compounds which may produce scale on the metal walls of the 
reaction apparatus or cause crevice and stress corrosion, resulting in 
cracking, spalling and ultimately catastrophic failure of the reaction 
apparatus. By removing these corrosive and scale-producing components from 
the waste before wet oxidation, less expensive metals may be used to 
construct wet oxidation reaction apparatus 58 and less frequent cleaning 
of the walls of reaction apparatus 58 is needed. Corrosion is further 
reduced in the present invention by using a low chloride feed diluent 
which is preferably derived internally in the manner described. As stated, 
calcium and other metals forming insoluble carbonates may also be removed 
by treating the effluent 22 with effluent gas. This decreases scaling even 
further. 
The method of wet oxidation and the preferred wet oxidation apparatus 58 
used in the present invention uses the principles disclosed in U.S. Pat. 
No. 4,272,383 to McGrew which is assigned to the assignee of the present 
invention and which is incorporated herein by reference. The wet oxidation 
reaction apparatus disclosed therein includes an assembly of vertically 
disposed concentric tubes nested to form an influent waste passage and an 
effluent waste annulus. The reaction vessel extends vertically 
approximately one mile below ground and is generally known as a down-hole 
wet oxidation reaction system. Diluted liquid waste 56 is flowed into the 
influent passage as an influent waste stream. Diluted liquid waste 56 is 
heated by a counter current heat exchanger as it flows into the wet 
oxidation reaction apparatus 58, and a source of oxygen 60, such as air or 
an oxygen-rich gas, or pure oxygen, is mixed therewith so that the flow 
inside reaction apparatus 58 provides good contacting and mixing between 
liquid waste 56 and oxygen 60. In the most preferred embodiment of the 
present invention, liquid waste stream 56 is heated using waste heat 
recovered in the energy recovery unit including biogas 38 derived from the 
biological polishing unit 36 as fuel. The oxygenated liquid waste forms a 
hydrostatic column of considerable fluid pressure in the influent waste 
passage where the exothermic wet oxidation reaction continues, generating 
substantial heat. Between 1,000 and 6,000 feet below ground surface, a 
reaction zone is established where the temperature of the liquid waste 
reaches approximately 530.degree.-550.degree. F., producing a 
self-sustaining, greatly accelerated wet oxidation reaction. Boiling of 
the material at this high temperature is inhibited by the hydrostatic 
fluid pressure of the column. 
Although in our preferred method diluted waste 56 is treated by wet 
oxidation to oxidize the combustible waste components, the wet oxidation 
step of the present invention may be replaced with some type of anoxic 
thermal conditioning process and may include hydrolysis or the like. As 
set forth in the aforementioned McGrew patent, substantial heat can be 
recovered from the wet oxidation reaction of the preferred method of the 
present invention which can be advantageously used to heat the liquid 
influent waste stream. 
Next, the effluent 62 from the wet oxidation reaction apparatus 58 is 
flowed to ash solids/liquid separation equipment 64 to further concentrate 
the ash solids. This solids/liquid separation step preferably includes 
clarification, thickening, by a plate separator and then dewatering to 
form sterile ash 22 and liquid stream 66. A gravity settler or dissolved 
air flotation unit could be used in lieu of the plate separator. As 
stated, at least a portion of the resulting sterile ash 22 is used as a 
body feed or weighting agent to promote sedimentation of the raw waste 20. 
In addition to its ability to promote settling of raw waste 20 during the 
initial dewatering step 26, any COD remaining in ash 22 is further reduced 
in its second trip through the wet oxidation apparatus 58. Liquid stream 
66 is flowed to the biological polishing unit 36 for further treatment in 
the described manner. It may be suitable and necessary in some instances 
to first strip liquid effluent stream 66 of ammonia. However, as stated, 
the concentration of ammonia is very low at this stage due to the initial 
dewatering step of the present invention. Of course, as stated, this 
initial dewatering step substantially reduces the amount of ammonia which 
passes through the wet oxidation reaction apparatus, thus reducing the 
production of undesired nitrogen-containing effluent products such as 
pyridenes and other nitrogen-containing organic compounds. 
It is to be understood that in its broadest scope, the present invention 
comprehends separating a substantially untreated waste to form a liquid 
portion and a solid portion using an internally-derived ash as a weighting 
agent as previously described. It is preferred that the liquid portion so 
isolated then be processed by the preferred method of the present 
invention. 
Referring now to FIG. 3 of the drawings, in another embodiment of the 
present invention, raw substantially unprocessed waste 100 is diluted 
preferably with an internally-derived diluent 102 which has been 
substantially desalinated. It may be possible to use a desalinated 
externally derived diluent alone or in combination with internally-derived 
diluent 102. Waste 100 is diluted with diluent 102 in mixer 104, and the 
resulting diluted waste 106 is flowed directly to wet oxidation apparatus 
108 where it is combined with gaseous oxygen or an oxygen-rich gas 110. 
Diluted waste 106 preferably is diluted such that solids are present in a 
concentration of about 5% by weight. In wet oxidation reaction apparatus 
108, combustible waste present in diluted waste 106 is substantially 
oxidized. Again, the preferred method and apparatus for carrying out the 
wet oxidation reaction uses the principles disclosed in the McGrew patent, 
which was previously described. The reaction products or effluent 112, 
produced during the wet oxidation reaction, are then flowed to separation 
equipment 114 where effluent 112 is degassed when the liquid portion is 
separated from the solids. The off gases 116 include carbon dioxide. 
Solids portion 117 comprises a low-volume sterile ash. The liquid effluent 
portion 118 is then preferably further processed beginning with the 
removal of ammonia and certain anions including phosphate, sulfate, 
carbonate and oxalate ions. A part of liquid portion 118 may also be 
flowed directly back to mixer 104 to be used in diluting waste 100. It is 
preferred, however, that the liquid portion 118 receive further treatment 
in the manner to be described. 
Liquid portion 118 is flowed into ammonia stripper/precipitator 120 where 
it is combined with precipitants such as calcium carbonate 121. As 
previously described, the addition of calcium carbonate is controlled such 
that phosphate, sulfate, carbonate and oxalate ions are precipitated out 
of solution and the pH of liquid portion 118 is sufficiently raised such 
that ammonia is driven off as a vapor to be condensed for use as a 
fertilizer 122. A liquid stream 124 is thus produced which has a 
substantially reduced ammonia content and from which the foregoing anions 
have been substantially removed. Sufficient nitrogen compounds are left, 
however, in liquid stream 124 to facilitate subsequent biological 
polishing. 
Liquid stream 124 is then flowed into precipitator 126 where it is mixed 
with carbon dioxide from the off gases 116 derived during the degassing of 
the wet oxidation effluent 112. Any potential scale-forming ions remaining 
in liquid stream 124, such as calcium and magnesium ions, are thereby 
carbonated. The resulting de-ammoniated, deionized stream 128 is then 
flowed into biological polishing unit 130 where anaerobic microbes 
decompose much of the dissolved and suspended organic matter. Biological 
polishing 130 may include a sequence of anaerobic polishing followed by 
aerobic polishing if desired in a particular application. Biogas 131 is 
evolved as a byproduct of the biological polishing process and includes 
methane and carbon dioxide. It may be appropriate, as in the previously 
described embodiment, to use biogas 131 to carbonate liquid stream 124 to 
remove magnesium and calcium ions. By stripping out the carbon dioxide in 
this manner, the fuel value of biogas 131 is increased due to the 
resultant increase in the percentage of methane. Sludge or biomass 132, 
which is also generated during the biological polishing step, is combined 
with the diluted waste in mixer 104 to be oxidized in wet oxidation 
apparatus 108. 
Liquid stream 134, resulting after biological polishing 130, has a 
substantially reduced COD. Preferably, liquid stream 134 receives further 
processing by filtration unit 136, which is preferably an ultrafiltration 
system. The filtered stream 138 is then treated by reverse osmosis 
treatment 140, which produces diluent liquid stream 102 that is 
substantially free of chlorides, magnesium, calcium, phosphate, sulfate, 
carbonate and oxalate. The removal of chlorides, magnesium, and calcium 
are particularly significant due to their tendency to corrode metal 
surfaces as previously indicated. The deionized or demineralized liquid 
stream 102 is then preferably used as a diluent for water 100 as 
described. As in the previous embodiment, filtration 136 and reverse 
osmosis 140 may be replaced by evaporation means (not shown). Of course, 
rather than using liquid stream 102 as a diluent, it may be simply 
discharged into receiving waters. 
Having described the preferred method of the present invention, it is to be 
understood that various modifications may be made to the invention 
disclosed herein within the purview of the appended claims.