PIG SLURRY TREATMENT STRATEGY IN A HIGH LIVESTOCK CONCENTRATION AREA: ANAEROBIC DIGESTION AS THE KEY PROCESS

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1 PIG SLURRY TREATMENT STRATEGY IN A HIGH LIVESTOCK CONCENTRATION AREA: ANAEROBIC DIGESTION AS THE KEY PROCESS A. Bonmtí 1 nd X. Flotts 2 1 Lortory of Chemicl Engineering nd Environment. University of Giron, Cmpus Montilivi s/n, 1771 Giron, Spin, E-mil:.onmti@lequi.udg.es 2 Lortory of Environmentl Engineering, Dpmt. of Environment nd Soil Science. University of Lleid, Rovir Roure, Lleid, Spin, E-mil: flotts@mcs.udl.es Astrct A pig slurry tretment strtegy must include processes with the im of reducing voltile orgnic compound emissions, controlling odours, minerlising nutrients, improving its fertilising proprieties, nd recovering energy, when it is possile. In this sense, neroic digestion fulfils ll this requirements. However, in res with high niml frming density nd structurl nutrient surplus, it is necessry to include other processes, which fvour nutrient redistriution. The ojective of this pper is to study the importnce of including neroic digestion process in the tretment strtegy when the min ojective is to recover nitrogen y mens of mmoni ir stripping t 8ºC, or y vcuum evportion in order to reduce volume nd to fvour redistriution. Two types of pig slurry (fresh nd neroiclly digested slurry) were used nd the initil ph ws chosen s the control vrile. Aprt from producing prt of the required therml energy, previous neroic digestion presented severl cler dvntges: The consumption of voltile ftty cids nd voltile orgnic compounds during neroic digestion reduced the voltilistion of orgnic mtter in the stripping nd vcuum evportion processes. This fct resulted in high qulity condenste when vcuum evportion is pplied nd n mmoni slt with low orgnic mtter contmintion when nitrogen is recovered with stripping / sorption process. In this cse, high mmoni removl efficiency is possile, over 96%, without modifying the ph. Keywords Aneroic digestion, pig slurry, pre-tretment, post-tretment, mmoni stripping, vcuum evportion Introduction A tretment strtegy of given type of wste is comintion of severl processes with the ojective of otining profitle products, recovering energy nd rw mterils nd minimizing negtive environmentl effects. The strtegy definition depends on the composition of the wste, on locl circumstnces, nd on the required qulity for the end products. 424

2 For pig slurry, the inclusion of neroic digestion in the tretment strtegy offers severl dvntges: preventing voltile orgnic compound emissions, controlling odours, minerlising nutrient nd recovering energy through methne production. However, in geogrphicl res with high niml frming density supplementry process with the im of reducing volume nd/or recovering nutrients hs ecme need. It should e lso tken into ccount tht when energy cogenertion is pplied, the fesiility of the process is lso dependent on profitle uses of the recovered hed. Profitle uses of this energy frction re limiting fctor in wrm countries. The use of the surplus therml energy for pre-tretments, in order to improve neroic digestion, hs een demonstrted s good wy of promoting the economic nd energetic dvntges of neroic digestion implementtion nd opertion (Bonmtí et l., 21) Since nitrogen is the nutrient tht generlly limits the mount of mnure tht cn e pplied to the soil, nitrogen removl nd/or recovery is relevnt for improving mngement in res with structurl nitrogen surplus. Tretments for recovering nitrogen from livestock wste re pproprite since they tend to close the nitrogen cycle (llowing the recovered product to e recycled into the griculturl system) nd therey to sve energy. In this sense, ir stripping in comintion with sorption could e nd interesting option. Another strtegy for improving pig slurry mngement is its redistriution etween res with nutrient surplus nd those with shortge. However, this is limited y trnsporttion nd spreding costs due to the high wter content of the slurries nd their reltively low nutrient concentrtion. Tretments s vcuum evportion, which im to reduce volume, economiclly fvour redistriution. The existence of chep source of therml energy is proly the min limittion in the cse of evportion. In this sense, the Spnish Royl Decree 2818/98 fvours economiclly the use of wste het from electricl power plnts for orgnic wste volume reduction. In the present pper, it will e nlysed the importnce of the inclusion of neroic digestion in the tretment strtegy when the min ojective is to recover nitrogen, pplying mmoni ir stripping, or to reduce volume y vcuum evportion for fvouring redistriution. Two flow digrms of nitrogen recovery strtegy were compred, including or not neroic digestion (Figure 1). ) ) Ammoni Slt CH 4 CO 2 Rw Pig Slurry Ammoni Air Stripping/ Asorption Treted Pig Slurry Rw Pig Slurry Aneroic Digestion Ammoni Air Stripping/ Asorption Treted Pig Slurry Ammoni Slt Figure 1 Nitrogen recovery strtegy: ) mmoni ir stripping nd ) neroic digestion nd mmoni ir stripping. 425

3 As well s in nitrogen recovery strtegy, two flow digrms were compred in the reduction of volume strtegy (Figure 2). ) ) Treted Pig Slurry CH 4 CO 2 Rw Pig Slurry Vcuum Evportion Condenste (wter) Rw Pig Slurry Aneroic Digestion Vcuum Evportion Treted Pig Slurry Condenste (wter) Figure 2 Reduction of volume strtegy: ) vcuum evportion nd ) neroic digestion nd vcuum evportion. Mteril nd methods Two different experiment were conduced: mmoni ir stripping / sorption experiments nd vcuum evportion experiments. Ammoni ir Stripping / Asorption experiments A semi-continuous l scle mmoni ir stripping / sorption system ws constructed n operted (Bonmtí nd Flotts, 22), in order to compre the fesiility of mmoni ir stripping s pre- or post-tretment to mesophilic neroic digestion. Two different pig slurry (fresh pig slurry nd neroic digested slurry, see Tle 1) were used t three different initil phs (non-modified, 9.5 nd 11.5). ph djustment ws mde y clcium hydroxide. Pig slurry ws previously filtered through 2 µm sieve. Temperture ws set t 8ºC. Sulphuric cid ws used in the sorption process. The fesiility of the opertion (% mmoni removed nd the removl rte) nd the chrcteristics of the mmoni slt otined were used to compre the tretment strtegies. Tle 1 Chrcteristion of the filtered pig slurry used in mmoni ir stripping experiments (verge of three smples) ph TS VS COD NH 4 + -N TKN VFA Fresh Slurry Digested Slurry

4 Vcuum evportion experiments The inclusion of neroic digestion previous to the evportion process ws studied y mens of the chrcteristic of the condenste recovered in tch l scle evportor (Bonmtí nd Flotts, 22). As in ir stripping experiments, two different slurry types were used (fresh pig slurry nd neroiclly digested slurry, see Tle 2) nd three initil phs (4, 5, nd 6). ph djustment ws mde y sulphuric cid. Pig slurry ws lso filtered through 2 µm sieve. Temperture ws set t 4ºC nd pressure t 5 mmhg. Tle 2 Chrcteristion of the filtered pig slurry used in vcuum evportion experiments (verge of three smples) ph TS VS COD NH 4 + -N TKN VFA Fresh Slurry Digested Slurry Anlyticl methods Totl Kjeldhl nitrogen (TKN), mmoni nitrogen, ph, totl solids (TS), voltile solids (VS), nd chemicl oxygen demnd (COD) were ll nlysed y stndrd methods (APHA, 1995). Voltile ftty cids (VFA) were nlysed y cpillry gs chromtogrphy with Flme Ionised Detector (FID). Sttisticl methods Sttisticl nlysis ws performed using SAS softwre (SAS Institute, 1989). A one wy ANOVA test ws crried out. When this nlysis indicted significnt differences nd interction ws significnt, Lst Squre Mens test ws performed with significnce level of 5%. Results nd discussion Ammoni ir Stripping / Asorption experiments Ammoni removl efficiencies re presented in Tle 3. As it cn e seen, in ll cses percent mmoni removl is greter in experiments with digested slurry thn those with fresh pig slurry, except when initil ph ws set t Finl mmoni concentrtions elow.12 g/kg, nd mmoni removl efficiencies ove 96%, were reported in ll tretments with digested slurry. It is possile to completely remove mmoni without ph modifiction when using digested slurry. This ws in ccordnce with Collivignrelli et l. (1998), who reported experiments performed with n old lndfill lechte showing tht it ws possile to chieve high mmoni removl efficiencies without se dosge, if temperture ws mintined t etween 6 7ºC. 427

5 The different ehviour of the two slurry types could e ttriutle to the different concentrtion in VFA (Tle 1). The high content of VFA in fresh slurry leds to fll in ph during the stripping process. A ph decrese of etween ws reported in fresh slurry experiments. This fll in ph results in lower mmoni removl efficiencies in fresh slurry experiments thn in digested slurry experiments. Low phs reduced mmoni voltility y shifting its dissocition rection towrds the ionic form (Perry, 1992). Tle 1 Ammoni removl efficiency (%) in mmoni ir stripping experiments Initil ph Non-modified % Ammoni removl Fresh slurry Digested slurry As seen in Figure 3, the different ph levels hd different effects on mmoni removl rtes, ccording to the sustrte. In the experiments performed with fresh pig slurry, lthough the initil ph ws set t 9.5, mmoni removl rtes did not increse significntly (letters -c in Figure 3). A high initil ph (11.5) ws necessry to otin significnt increse in the mmoni removl rte. The neroiclly digested slurry showed different ehviour. On seting the initil ph t 9.5 the mmoni removl rte incresed significntly, ut no further increse ws reported when initil ph ws set t x k (h-1) x y x y y. Non Modif ph=9.5 ph=11.5 Digest Slurry Fresh Slurry Figure 3 Ammoni removl rtes for the different phs nd sustrte types. Vlues with the sme letter re not sttisticlly different t 5% level of significnce. Letters to c re used to compre initil phs for the sme sustrte, letters x to z re used to compre sustrtes with the sme initil ph The wter contining mmoni slt otined in the sorption process ws crystllized. The chrcteristics (colour nd COD) of the crystllized mmoni slt differ etween the two slurry types. The higher COD of the wter mmoni slts coming from the fresh pig slurry resulted in drker colour thn tht from digested slurry. 428

6 These results showed tht mmoni ir stripping hs mny dvntges if it is performed s post-tretment to pig slurry neroic digestion. It is possile to chieve high mmoni removl efficiencies without ph modifiction nd COD low contmintes the mmoni-slt wter otined in the sorption process. Vcuum evportion experiments Condenste chrcteristics s function of the initil ph re showed in Figure 4. As expected, the higher the initil slurry ph, the higher mmoni concentrtion in the condenste, nd the lower the initil ph, the higher totl voltile ftty cids (VFA) concentrtion in the condenste. This is in ccordnce with theoreticl studies performed y Mrks et l. (1994). Both types of slurry present the sme pttern. COD present different ehviour depending on the type of slurry. In fresh slurry experiments, COD in the condenste incresed when ph decresed. The COD ssocited with VFA represented 27% of the totl COD. On the other hnd, in experiments with digested slurry, no reltionship ws found etween COD nd the initil ph. This suggests tht the voltile orgnic mtter of the digested slurry ws minly composed of non-ionised compounds, nd ph did not ffect its voltilistion. The VFA concentrtion of the slurry only represented the 1.5% of the totl COD. It should lso e noted tht, t given ph, mmoni nitrogen, COD nd VFA concentrtion in the condenste from fresh slurry were higher thn those from digested slurry (Figure 4). This is in ccordnce with the higher mmoni nitrogen, COD nd TVFA concentrtion of the fresh slurry thn the digested slurry (Tle 2). 12 () Fresh Slurry 12 2 () Digested Slurry 7 COD nd TVFA (mg/l) NH 4 + -N (mg/l) COD nd TVFA (mg/l) NH 4 + -N (mg/l) Initil slurry ph Initil slurry ph COD VFA NH4+-N Figure 4 Ammoni nitrogen, COD nd VFA concentrtion on the condenste s function of the initil slurry ph: () experiments with fresh slurry, () experiments with digested slurry In order to void the effect of the initil slurry concentrtion nd to compre the ehviour of the two slurry types, the percent mss trnsferred to the condenste ws clculted. 429

7 The percent of voltile ftty cids trnsferred to the condenste ws higher in experiments with fresh slurry thn those with digested slurry (Figure 5). In contrst, percent mmoni nitrogen trnsferred to the condenste in experiments with digested slurry ws lrger thn in those with fresh slurry, for ll initils phs tested (Figure 5). This fct cn e explined y the finl ph of the concentrte. An increse of etween.3 nd 1.9 ws oserved depending on the initil ph nd the type of slurry involved. This ph increse ws higher in the experiment performed with digested slurry thn in those with fresh slurry. This showed tht when using fixed initil slurry ph, the frction of un-ionised VFA nd mmoni nitrogen of the two slurry types were different, nd explined the differences oserved in the percentges of the compounds trnsferred. In ll cses, the COD% trnsferred to the condenste in the experiment with fresh slurry ws lrger thn tht reported in those with digested slurry (Figure 5c). The different nture of the orgnic mtter of the slurries my explin this fct. It cn e concluded tht under these opertionl conditions nd within this rnge of ph vlues, one-stge evportion tretment would e sufficient to otin high qulity condenstes if previous neroic process is pplied. NH4 + -N trnsferred to condenste (%) ph =4 ph =5 ph =6 TVFA trnsferred to condenste (%) ph =4 ph =5 ph =6 COD trnsferred to condenste (%) ph =4 ph =5 ph =6 c Fresh Slurry Digested Slurry Figure 5 Ammoni nitrogen (), VFA () nd COD (c) trnsferred (%) to the condenste in the tch evportion experiments. With the sme initil slurry ph, different letters show significnt differences etween mens (5% significnce) 43

8 Conclusions Previous neroic digestion of the slurry presents cler dvntges for mmoni ir stripping nd vcuum evportion. A prt of providing prt of the therml energy needed for the studied processes, the conversion into CH 4 nd CO 2 of most of the voltile orgnic improves the glol tretment opertion. It is possile to chieve high mmoni removl efficiencies without se dosge nd the mmoni slt otined in the stripping process is less contmintes when slurry hs een previously neroiclly digested. The otined condenstes in the evportion process lso present lower concentrtions of COD nd VFA when previous neroic digestion process is performed. Those results showed tht neroic digestion is necessry step in glol pig slurry tretment strtegy. A prt of the well know enefits of neroic digestion, the inclusion in the tretment strtegy improves the tretment opertion, nd the qulity of the end products otined. References Bonmtí, A. nd Flotts, X. (22), Air stripping of mmoni from pig slurry chrcteristion nd fesiility s pre- or post-tretment to mesophilic neroic digestion. Wste Mngement. Sumitted. Bonmtí, A. nd Flotts, X. (22). Pig slurry concentrtion y vcuum evportion: influence of previous mesophilic neroic digestion process. Journl of the ir & wste mngement ssocition. Sumitted. Bonmtí, A., Flotts, X., Mteu, L, nd Cmpos, E. (21). Study of therml hydrolysis s pre-tretment to mesophilic neroic digestion of pig slurry. Wt. Sci. Tech., 44 (4), Collivignrelli, C., Bertnz, G., Bldi, M. nd Avezzù, F. (1998). Ammoni stripping from MSW lndfill lechte in ule rectors: process modelling nd optimiztion. Wste Mnge. Res., 16 (5), Mrks, A.L., Luthy, R.G. nd Diweker, U.M. (1994). Semi-continuous evportion model for lechte tretment process evlution. Environmentl Progress, 13 (4), Perry R.H. Mnul del ingeniero químico. 6th edn., McGrwHill, 1992; Vol. 1. SAS Institute. SAS/STAT User s guide. Version 6. 4rth ed. SAS Institute, Cry, USA, Stndrd Methods for the Exmintion of Wter nd Wste Wter (1995). 19th edn., Americn Pulic Helth Assocition/Americn Wter Works Assocition/Wter Environment Federtion, Wshington DC, USA. 431