This invention concerns a process for slurry deodorisation and especially the deodorisation of slurry produced by animals

Farmyard animals naturally produce substantial volumes of waste material which is converted into aqueous slurries that are stored in pits or tanks before their disposal Such slurries are often malodourous, on account of sulphidic or other noxious or unpleasantly smelling gasses that emanate from the slurries. Slurries contain significant nutrients for plant and a long standing method of their disposal comprises spreading them as a fertiliser to pasture and arable land prior to or during crop production

The problem has become more noticeable as a result of modern developments in animal husbandry and in particular by the tendency towards employing more intensive units for rearing farmyard animals such as pigs or cattle and the trend for the numbers of animal per unit to increase Accordingly, the volumes of slurry to be treated have grown Moreover, the urbanisation of villages and their transformation into dormitories for nearby towns and cities has introduced former town dwellers into closer proximity to the smells of the countryside, including odorous animal slurries, resulting in an increase in complaints to farmers about offensive treatable smells which long- standing village dwellers have ignored previously In addition, a number of countries have introduced new or more stringent regulations on the control of emissions into the atmosphere to prevent nuisance to neighbouring communities or for improving the health and general safety of those persons working in the vicinity As a consequence, there is a continuing and increasing need for treatments to remove malodours from animal slurries. Storage and handling of animal waste slurries also presents hazards wrt toxic gas emission - namely hydrogen sulphide, indeed several deaths have occurred in the EU farming community attributable to manipulation of anaerobic slurry

waste In particular procedures involving acidification of anaerobic waste slurry to enhance the slurry fertilizer value via nitrogen fixation are particularly hazardous as toxic hydrogen sulphide gas is evolved during the acidification process. It has hitherto been suggested to employ a peracid composition for the treatment of animal slurries to remove odour, ie to ameliorate or eliminate the level of odour in the vicinity of the slurry For example in EP-A-062831 8 to SEPPIC, there is described a process for deodorising such slurries in which the slurry is treated with a special composition containing not only the peracid and hydrogen peroxide but additionally a mineral acid, with the result that the slurry is brought virtually instantaneously to its discharge pH Such a process according lacks a degree of flexibility in that deodorisation takes place at the discharge pH Such a process is consistent with the deodorisation process described in EP-A-0370565 to Kemira, in which the slurry is first adjusted to its discharge pH and then an oxidant is added, specifically hydrogen peroxide. In GB-A-1 583622 / USP 41 60656 to Degussa, it is alleged that in tests of deodorisation, oxidation chemicals by themselves, such as peracetic acid, do not have the required effect of suppressing irritant and noxious gases emitted from animal wastes. Consequently, Degussa suggested the employment of a composition containing both formaldehyde and the oxidant Such compositions disadvantageously and on Degussa's own admission, have a comparatively short storage life resulting from oxidation of the formaldehyde by the oxidant. Moreover, formaldehyde is itself a relatively toxic compound, a confirmed carcinogen and an irritant to eyes at an air concentration of as low as 20 ppm Its application into tanks and pits that are open to the atmosphere increasingly fall foul of health and safety regulations

It is an object of the present invention to provide a process for deodorising animal slurries which avoids or ameliorates the disadvantages indicated previously herein. According to the present invention, there is provided a process for deodorising animal slurry in which the slurry is brought into contact with an aqueous solution of a peracid characterised in that the deodorisation is carried out in two steps, in which, in step ( 1 ), the peracid is introduced into the slurry, and the slurry pH remains at over pH 6.5 during consumption of the peracid and thereafter in step (2) when the peracid in the slurry has been substantially consumed, sufficient acid is introduced into the slurry to lower its pH to below 6.5.

By the adoption of a process according to the instant invention, it is possible to separate the step of oxidation of malodours from the step of pH adjustment, thereby not only introducing flexibility into the process but enabling both steps to be separately controlled and improving the safety of the acidification stage via significant reduction in the extent of toxic hydrogen sulphide gas emitted during acid addition Comparative trials using a substantially similar concentration of peracid introduced into the animal slurry with either sufficient mineral acid to attain the discharge pH straightaway or in the two stage process of the instant invention have shown a significant improvement in gaseous emissions from the two step process compared with using the premixed acid/peracid mixture Moreover, the invention process achieves its results without employing the carcinogenic irritant, formaldehyde, thereby providing an inherently safer process. The invention process is applicable to a wide range of slurries obtainable from the excrement of farmyard animals In this context, animal is employed in its loosest sense to include intensively reared poultry, such as chickens, turkeys or ducks. Many applicable slurries are derived from pig effluent can also be derived from cattle and less commonly from deer. The slurries can also contain leachate from excrement soiled bedding, for example from middens. The slurries are also obtainable from animals held in zoological gardens and particularly those located in city centres such as in Regent's Park, London

By virtue of their organic nitrogenous content the animal slurries before treatment often are alkaline, many having a pH in the region of from 7 to 8.5. The solids content of the slurries can vary over a wide range at the discretion of the slurry producer. In the majority of slurries, the solids content falls in the range of from 0.5 to 20% by weight.

The peracid introduced in step ( 1 ) can be selected over a wide range of peracids Suitable peracids include aliphatic monoperoxycarboxyhc acids and especially water soluble peracids containing from 1 to 6 carbons. Performic acid, though an effective material for deodorisation, usually requires on the spot generation. Higher homologues, C2-C6 peracids, such as peracetic acid or perpropionic acid can be prepared and stored, often for considerable periods of time before use. By virtue of its ready availability, in many embodiments of the instant invention peracetic acid is employed.

Other suitable peracids include aliphatic diperacids, such as diperoxysuccinic acid, diperoxyglutaric acid or diperoxyadipic acid and, though less desirably, aromatic peracids such as perbenzoic acid or monoperphthalic acid. Yet other suitable acids can comprise hydroxyaliphatic peracids such as peroxycitric acid, peroxytartaπc acid, and peroxygluconic acid. Still other suitable peracids include ester monoperacids, and specifically the ester peracids obtainable from C1 -C4 mono-esters of C3-C7 aliphatic dicarboxylic acids such as methyl monopersuccinic or monoperglutaric or monoperadipic acid or a mixture of the three ester peracids.

It will be recognised that the peracids described herein are often obtained by reaction between the corresponding carboxylic acid and hydrogen peroxide, sometimes in the presence of a catalyst to accelerate the rate of peracid formation. The peracid composition employed in step 1 herein usually comprises an aqueous solution. It normally includes at least some hydrogen peroxide and often an amount selected in the range of from 1 .5 to 50 moles per mole of peracid. In a number of embodiments, the hydrogen peroxide is present in a mole ratio to the peracid selected in the range of from 3:1 to 30: 1 and often at least 10: 1 . The concentration of peracid in the aqueous solution introduced in step 1 is often selected in the range of from 0.25 to 25% by weight and in many instances in the range of from 1 to 1 5% by weight. The concentration of hydrogen peroxide is often selected in the range of from 5 to 45% and in many instances from 10 to 35% by weight. The concentration of carboxylic acid in the composition is often selected in the range of from 2% to 30% by weight and in many instances from 3 to 20%.

The aliphatic monoperacids and diperacids, hydroxyperacids and ester peracids herein usually can form compositions in which the peracid, carboxylic acid, hydrogen peroxide and water are present in equilibrium amounts, governed by the equation RCO ₂ H + H ₂ O ₂ <-> RCO ₃ H + H ₂ O where R represents an organic moiety such as methyl. Equilibrium compositions can be obtained by mixing hydrogen peroxide and organic acid in a chosen ratio and in a selected weight of water and permitting the reaction to reach equilibrium or accelerating it by the presence of catalyst.

One preferred range of compositions for use herein comprises solutions containing by weight from 2.5 to 3.5% peracetic acid, 27 to 35% hydrogen peroxide, and 1 5 to 20% acetic acid Other useable compositions comprise solutions containing from 4 to 6% peracetic acid, 1 5 to 22% hydrogen peroxide, and 1 5 to 22% acetic acid, or 1 1 to 14% peracetic acid, 1 7 to 22% hydrogen peroxide and 3 to 7% acetic acid.

Other suitable compositions comprise solutions containing ester peracids, such as those mentioned hereinbefore, desirably to provide a peracid concentration, calculated as its active oxygen of from 1 to 5 % w/w. Such peracids can conveniently be made by mixing aqueous hydrogen peroxide with esters of C3 to C7 dicarboxylic acids, such as methyl esters of succinic acid, glutaπc acid and adipic acid or mixtures containing any two or all three of them, usually in the presence of a strong acid catalyst, and permitting the compositions to approach or reach equilibrium. The esters may be selected from monoesters or diesters of the dicarboxylic acid or mixtures of the acids Commercially available mixtures of esters presently contain from 0 to 20% suecinate, 50 to 80% glutarate and 1 5 to 30% adipate. Such compositions often contain from 5 to 20 % hydrogen peroxide and equilibrium concentration of hydrolysis and perhydrolysis derivatives of the ester peracids or ester starting material. The resultant peracids often comprise a mixture of monoester peracids and/or acid peracids in a ratio that depends on the process conditions and starting esters employed

The peracid is often introduced into the slurry to provide a concentration calculated as its active oxygen of from 0 5 to 25 ppm, in many instances from 1 to 1 6 ppm, and preferably from 1 .5 to 5 ppm. It will be recognised that a monoperacid provides 1 6g active oxygen per mole of peracid. The amount of peracid composition to deliver the desired concentration of peracid can be calculated readily, taking into account the concentration of peracid in the composition Thus, for example, it is particularly desirable to employ the preferred compositions containing from 2.5 to 3.5 % peracetic acid in an amount of from 200 to 800 g composition per cubic metre of slurry The comparatively small amount of peracid composition that is needed in order to provide the peracid concentration desired for odour control means in practice that it is usually convenient to introduce the peracid solution in a single dose, though multiple dosing could be used if desired .

It will be recognised that slurries of animal wastes usually contain a number of pathogenic microorganisms of animal and/or human origin and that by selecting an appropriate concentration of the peracid, it is possible to achieve a significant disinfection effect in addition to deodorisation, the extent of disinfection varying with peracid concentration, the resistance of the microorganism and subsisting operating conditions such as the organic material content of the slurry and the temperature. For example, viable populations of pathogenic bacteria such as Vibrio cholerae, Staphylococcus aureus, Escherichia coli, Streptococcus faecalis, Proteus vulgaris and Salmonella sp. can often be reduced by at least 90% to 99% or under favourable conditions even more, at peracid concentrations selected within or around the preferred concentration range for deodorisation, eg up to about 6 ppm peracid calculated as the active oxygen, and usually over 99% reduction at higher peracid concentrations, eg in the range of 6 to 25 ppm peracid calculated as its active oxygen. The actual peracid concentration employed may take into account the extent to which disinfection is desired as well as deodorisation. Under adverse local conditions, an even higher peracid concentration above 25 ppm as active oxygen such as up to 60 ppm can be contemplated. The peracid in step 1 can be introduced into the tanks and pits of slurry in the farmyards over an expended period with mixing to promote a distribution of the active constituent or constituents throughout the slurry. In many instances, the mixing takes from 5 to 30 minutes to achieve a reasonably acceptable extent of mixing. As a result of the introduction of relative small amount of the peracid composition, there is usually no detectable change in the pH of the slurry, so that it still remains at above pH 6.5 and in most treatments remains above pH 7 during the course of the deodorisation step 1 .

Step 1 is usually continued after introduction of the peracid until there is substantially no detectable peracid in the slurry and advantageously no residual hydrogen peroxide either A convenient way of detecting the presence or absence of peracids and hydrogen peroxide is to employ testing strips such as a MERKOQUANT™ strips. Detection strips for peracetic acid and hydrogen peroxide turn blue in the presence of peracid or hydrogen peroxide and remain colourless in their absence. It has been found that the peracid reacts very quickly in the slurry and is in many instances substantially consumed within a few minutes after its introduction is completed, for example within 2 to 10 minutes. In practice, it is often

convenient to determine in a trial run the period of time for the peracid to be substantially consumed and thereafter to employ the determined period, often plus a safety margin such as of at least about 5 minutes before step 2 commences. Although the treatment in step 1 is very quick, its effect of significant control of odorous gas emissions has been observed to remain for at least a day when dosed to an effective level for immediate lowering of such gas emissions from a particular slurry.

In step 2, an acid solution is introduced into and mixed with the slurry to bring its pH to below pH 6.5, usually to a pH not below pH 4 and in many instances to bring it to within the range of from pH 5 to 6.2.

The acid in step 2 can be selected from a wide range of acids, ie materials having a pK _a of lower than and preferably at least 1 unit lower than the pH to be attained by the slurry in step 2. The acid in step 2 of many convenient treatments comprises a mineral acid such as sulphuric phosphoric and/or nitric acid and it is especially convenient to employ sulphuric acid. Where it is desired to increase the nitrogenous or phosphorus content of the slurry for eventual distribution as a fertiliser on fields, one convenient way comprises the use of at least a proportion of nitric and/or phosphoric acids in the acidification. The concentration of the mineral acid is at the discretion of the process user. Commercially available concentrated mineral acid solutions, such as from 70 to 98% w/w sulphuric acid, can be used without dilution and more dilute or diluted mineral acids can be used, if desired, such as from about 1 5 to 30% w/w. . Alternatively, the acid can comprise wholly or partly an organic acid, such as an aliphatic or hydroxyaiiphatic carboxylic acid of which acetic acid, propionic acid, lactic acid, citric acid and tartaric acid are suitable examples. At least a proportion of the acid can be provided by acidic media generated on farms, such as silage effluent.

The amount of acid to employ in step 2 is determined by such factors as its strength, the pH of the slurry before step 2, the pH it is desired to attain after step 2 and the solids density and nature of solids in the slurry. In many instances it is convenient to determine how much of the selected acid to employ by dosing to reach the preselected final pH, either on a sample or in the treatment tank or pit. Thereafter, the same amount can be employed to attain substantially the same pH on future batches until the slurry characteristics change. In a number of treatments, the amount of acid employed (acid calculated as 100% H ₂ SO ₄ ) falls within the range of from 50 to 500 Kg per m ³ slurry. The change in pH of the slurry is

virtually instantaneous on its contact with the acid, so that the duration of step 2 is often governed by the time taken for the added acid to be distributed through the slurry by the agitator or pump. In many instances, the mixing takes from 5 to 30 minutes to achieve a reasonably acceptable extent of mixing. The acid can be introduced over the mixing period or can be introduced in a single shot.

By virtue of the acidification step 2, a higher proportion of nitrogen in the slurry is fixed than would have been the case if no acidification had occurred. The slurry can be treated in both steps 1 and 2 within a wide range of temperatures at which the slurry remains mobile, such as from 5 to 30°C. Such a range encompasses its ambient temperature for most of the year of slurries produced in temperate zones such as Western Europe, USA and Japan. The slurry is often treated in tanks or pits having respectively walls or a lining made from corrosion protected galvanised steel, concrete, vitreous enamelled steel, various plastics or glass reinforced plastic. It is highly desirable for the treatment tanks or pits to be open or vented to the atmosphere so as to prevent or minimise the risk of a build-up of gasses and/or excess pressure above the slurry. The slurry in the tanks is agitated during and usually throughout the treatment process steps, for example with an agitator or a submerged pump, or by pumping the slurry through a recycle. Advantageously by virtue of the invention process, the emission of odorous or noxious gasses such as H ₂ S can be reduced and controlled. However, process operators should continue to take precautions to prevent them from breathing the atmosphere directly above the slurry, such as by wearing suitable breathing apparatus.

It is highly desirable for the equipment and lines used to store and transfer the peracid solution into the treatment vessel to be resistant to acid and the oxidant. Suitable materials for lines and storage vessels include selected grades of stainless steel, aluminium, PVDF(polyvinylidene fluoride), PTFE (polytetrafluoroethylene), HDPE (high density polyethylene) and PVC(polyvinylchloride), the choice being made locally and depending upon local storage and climatic conditions Advantageously, the two step process according to the present invention combines the advantage of obtaining enhanced deodorisation by conducting step 1 at a higher pH than step 2 with the advantage of flexibility in conducting both steps separately and of obtaining a slurry

having a desired proportion of its nitrogen fixed with reduced H ₂ S emissions resulting in enhanced safety for the process operator and others in the vicinity of the slurry.

Having described the invention in general terms, specific embodiments thereof are described in greater detail by way of example only.

In the Examples and Comparisons, the peracid compositions employed were as follows:-

PAA represents peracetic acid AA represents acetic acid

MEP represents a mixture of ester and derivative peracids derived by mixing a blend of dicarboxylic acid esters (100g, 1 6%, suecinate, 58% glutarate, 26% adipate) with hydrogen peroxide solution (23.6g, aq 85 % w/w) and sulphuric acid and allowing it to reach equilibrium having a peracid avox of 2.71 % w/w.

ME represents the remainder of the starting material and carboxylic acid derivatives of both MEP and the starting material.

In the Examples, a sample of pig slurry (500 mis), was transferred via a siphon pump into a polyethylene bottle whose mouth was fitted with a sealable flexible thin polyethylene tube and was subjected to sequential treatments with a peracid and then an acid.

In Comparisons labelled control runs, either the peracid or acidification treatment was not employed, or neither of them. In the other Comparisons, the same quantities of the same peracid solutions and of the titred amount of acid as in the Examples were introduced into the slurry as a premix, in a one step combined acidification and deodorisation process with the same consumption of reagents as the two step invention process. In the Examples, the treatments were carried out in two consecutive steps. In step 1 , the identified peracid composition in the text or Table was introduced via a pipette over several seconds to provide the peracid

concentration in the slurry calculated as its active oxygen (Avox - in mg 0 ₂ per litre)) content shown in the summary Tables. After 2 minutes MERKOQUANT ™ strip for residual peracid detection was inserted. No blue colouration of the strip developed indicating that there was substantially no residual peracid in the slurry. The slurry was then acidified in step 2 until pH 6 was reached using an aqueous sulphuric acid solution (25% w/w H ₂ S0 ₄ by weight, 17-20 mis) . The slurry was then stored for 1 hour and the H ₂ S level in the headspace above the slurry in the jar was measured using a Drager tube inserted through the polyethylene tube.

The results are summarised in the Tables below.

Testing similarly for ammonia during the course of the experimentation confirmed that the two step process substantially reduced the concentration of ammonia in the head space by over 90% compared with an untreated slurry.

Examples 1 and 2 and Comparisons CA to CD In these Examples and Comparisons, the slurries employed had a solids content of between 2 and 8% dry weight and pH of between 7.35 and 7.8 The results quoted in Table 2 are average results from several slurry samples, having solids dry weight within the 2-8% range, each measurement being carried out in duplicate.

Table 2

From Table 2, and specifically comparing CC with Ex1 and CD with Ex2, it can be seen clearly that the employment of a two step process reduced the level of H ₂ S emitted from the slurry more effectively than the corresponding single step process.

Examples 3 and 4 and Comparisons CE to CH

These trials were carried out in the standard manner using the same sample of pig slurry. Process variations and the results (averages of duplicate treatments/controls) are summarised in Table 3 below. Table 3

Peracid addition very quickly reduced the odorous gas emissions from treated slurry as compared to untreated or merely acidified slurry with the effect remaining for at least a day.

Example 5 and Comparisons Cl to CK

This Example and Comparisons were carried out on a 4% dry weight slurry sample initial pH 7.69. In Ex5 and CK, the slurry was treated with peracid composition S (peracetic acid) to provide a peracid concentration of 2.5ppm in terms of peracid Avox. The measurements of H ₂ S in the headspace were made immediately after completion of acid addition and at specified periods up to 24 hours after peracid addition. The results are summarised in Table 4.

^samples shaken before analysis

Examples Ex 6 to 9 and Comparisons CL and CM

In these Examples and Comparisons the slurry employed was a pig slurry which had a pH of 7.35 . The samples were treated two peracid compositions, dosed to provide the indicated peracid Avox level followed by pH adjustment to pH 6, with H ₂ S analysis of headspace gas one hour after reagent addition

Table 5

From Table 5, it can be seen that effectiveness of the deodorisation increased as peracid concentration increased and confirmed that the peracid solution S was more effective than solution O.

Examples Ex10 and Ex 1 1 and Comparison CN and CO

In these Examples and Comparisons, the slurry employed was a pig slurry which had a solids content of 4% dry weight and pH of 7.69 Samples were treated with respectively the ester peracid composition E or peracetic acid composition S to provide the same total Avox level of 66ppm (contributed to by the peracid and hydrogen peroxide present in the compositions) and H ₂ S levels were analysed at 1 hour after peracid addition and compared with control treatments. The results are summarised in Table 6. Table 6

From Table 6, it can be seen that the ester peracid composition E was also effective at reducing the concentration of H ₂ S in the headspace.