TREATMENT OF PIGGERY EFFLUENT

Abstract: A process for reducing the phosphorus content of piggery effluent involving the screening of the effluent to remove larger solids. Substantially clear liquid is then recycled as flushing liquid, the addition of fresh water being kept to a minimum. At periodic intervals, generally 24 hours, solid lime is added to the recycled liquid and fines to form a sludge. After settling clear liquid of a reduced phosphorus content may be discharged to waste or used for irrigation.

Description: TREATMENT OF PIGGERY EFFLUENT Field of Invention This invention relates to the treatment of piggery effluent . Background of the Invention Piggery effluent is generally high in phosphorus and if discharged without treatment will contaminate ground water, rivers, lakes and the like by encouraging the growth of algae and other aquatic plants. Various processes for treating the effluent have been considered but most are too costly to implement on a commercial scale. For example, raw effluent has been treated by the addition of lime but because of the quantity of lime required the cost is prohibitive. Description of the Invention It has been found that, by recycling the liquid content of the effluent as the flushing liquid for flushing the piggery and keeping the addition of fresh water to a minimum, the phosphorus content of the liquid portion of the effluent increases to saturation level. The phosphorus content of the solids also increases due to either precipitation of phosphorus from the saturated solution or by absorption of the organics in the solids or more likely a combination of both. After separation of the solids by screening and by settling portion of the supernatant liquid is recycled, the remaining supernatant liquid being held in the settling tank. The recycled liquid after passing through the piggery is again screened and the fines therein allowed to settle. At a suitable time solid lime is added to the settling tank and mixed with the fines. A lime sludge is formed and after settling thereof the liquid may be discharged or used for irrigation. The solids removed by the screening and settling and the lime sludge may be used as fertiliser. The quantity of lime required to treat the recycled effluent is consider¬ ably less than that required to treat non-recycled effluent. Thus in its broadest form the invention resides in a process for treating piggery effluent to reduce the phosphorus content comprising screening the effluent to remove the larger solid particles, separating the remaining fine solids by settling, recycling portion of the supernatant liquid through the piggery as flushing liquid whilst keeping the addition of fresh water to the system to a minimum, screening the recycled effluent to remove the larger solid particles, mixing the separated liquid and fines supernatant liquid with solid lime, separating the lime sludge formed and discharging the separated liquid to waste or irrigation. There will, of necessity, be some fresh water added to the system during recycling through wastage of water as the pigs drink and also from flushing of the pens when required. This should be kept to a minimum so that the flushing liquid remains saturated with phosphorus as much as possible. If too much fresh water is added to the system a portion of the phosphorus will be leached from the solids. Brief Description of the Drawings The invention will be better understood from the following description of the process as applied to a piggery having some 2000 pigs housed in two sheds which produce a total of approximately 27000 litres of effluent per 24 hour period and illustrated diagrammatically in the accompanying drawings wherein:- Fig.l is a diagrammatic representation of a first embodiment of the process; Fig.2 is a graph showing the results of treatment of liquid from various stages of the process with two different types of lime; and Fig.3 is a diagrammatic representation of a further process clarifying the liquid before discharge to waste. Detailed Description of Embodiiuen±.a In the embodiment shown in Fig.l the effluent from the sheds 11 is passed into a sump 12 from where it is passed through a screen 13 to remove the larger solids. The sepa¬ rated liquid with the entrained fines is then passed to a settling tank 14. The substantially clear supernatant liquid flowing over the top of the tank 14 is passed to a tank 15 from where it is recycled as flushing liquid. The recycled liquid again passes through the sump 12 and the screen 13 to remove any larger solid particles picked up during flushing. The separated liquid passes to the settling tank 14. The number of times the effluent liquid is recycled will be gov¬ erned by the size of the piggery and the amount of phosphorus can be retained by the solid particles. At suitable inter¬ vals, generally 24 hours, solid lime in the form of lime kiln dust or hydrated lime is added to the tank 14 and mixed with the liquid and fines therein. The mixture is passed to a settling tank 16. A lime sludge is formed and settles to the bottom of the tank 16. Substantially clear liquid is withdrawn from the vesseL 16 and discharged to waste or used for irrigation. The lime sludge is dewatered and dried and may be used as fertiliser. The larger solids removed by the screen 13 may be mixed with the lime sludge if desired, or used separately as a fertiliser. In order to establish the effectiveness of the process an estimated volume of piggery effluent was recycled through the piggery to mimic normal operating conditions for flushing and recycling. Lime kiln dust and hydrated lime was mixed with the final pass of the recycled liquid in the ratios set out in Table 1 as follows:- TABLE 1 Lime type Ratio Lime kiln dust 131:1 Lime kiln dust 88:1 Hydrated lime 743:1 Hydrated lime 391:1 The lime was added as it entered the vessel 14. A pump was used to mix the lime and effluent thoroughly and pump it to the settling tank 16 and allowed to settle overnight (16- 20 hours). Samples of the screened solids, primary waters (Prim- those which had been through the piggery once only), recycled (Pre primary waters which had been recycled through the piggery a number of times), and treated waters (Post- supernatant after settling) were taken and analysed for total phosphorus (TP) and total dissolved phospherus (DP). Total solid phosphorus was estimated by the difference. The results are set out in graph form in Fig.2 of the drawings. From a consideration of the graph it will be seen that the total phosphorus increased after recycling in each case according to the volume that was recycled (i.e. Prim to Pre). The increase was greatest where the least volume was recycled (i.e. LKD treatments) and least where the greatest volume was (i.e. HL treatments). Total dissolved phosphorus decreased as a result of each lime treatment (i.e. Pre to Post in Fig.2). Lime kiln dust was the most cost effective in terms of TP removal. Total dissolved phosphorus (DP) had the largest percentage decrease after treatment and always decreased by more than 93% as is shown in Table 2 below. Table 2 Percentage increase in phosphorus concentration for total phosphorus [TP] , total dissolved phosphorus [DP] and the total phosphorus due to solids [SP]. [TP]/( $/pig/annum) is the percentage decrease in TP per unit cost. Lime type [TPT [DPI [SP] [TP]/($/pig/ annum After chemical treatment the majority of P in the treated effluent was in solid form because of unsettled floes of lime. Solids removed by screening of primary waters contained 0.39% P. Recycling the primary waters resulted in an increase of the P content of the subsequent screened solids to 1.42% P. Recycling alone appears to contribute to the total reduction in phosphorus output. The process by which the P content of the screened solids increases could be one of absorption onto the organic matter. An unexpected bonus resulting from the process is that there is a reduction in the nitrate content of the treated effluent. The reason for this is not understood. Any unsettled floes of lime, which may contain phos¬ phorus and entrained in the liquid discharged from the settling tank 16, may be removed by utilising the embodiment of the process illustrated diagrammatically in Fig.3 of the drawings. The liquid passing from the settling tank 16 containing the floes of lime is mixed with sand, primary flocculant and polyelectrolyte in the vessel 17, utilising the techniques disclosed in Australian Patent Specification No.22673/88. The sand may be recycled.

Claims: CLAIMS 1. A process for treating piggery effluent to reduce the phosphorus content comprising screening the effluent to remove the larger solid particles, separating the remaining fine solids by settling, recycling portion of the supernatant liquid through the piggery as flushing liquid whilst keeping the addition of fresh water at a minimum, screening the recycled effluent to remove the larger solid particles, mixing the separated liquid and fines with solid lime, sepa- rating the lime sludge formed and discharging the separated liquid to waste or irrigation. 2. A process as claimed in Claim 1.wherein the solid lime is lime kiln dust or hydrated lime. 3. A process as claimed in Claim 1 wherein the liquid separated from the lime sludge is mixed with sand, flocoulant , and polyelectrolyte to remove any suspended floes of lime. 4. A process for treating piggery effluent to reduce the phosphorus content substantially as herein described.

Inventors: FITZPATRICK GREGORY PERCIVAL, FITZPATRICK, GREGORY, PERCIVAL
Applicants: FITZPATRICK GREGORY PERCIVAL, FITZPATRICK, GREGORY, PERCIVAL
Application year: 1991
Main classification: C02F1
IPC codes: A01C3, C02F1, C05F3