Applied Energy 120 (2014) 49–55

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Applied Energy

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Technical and operational feasibility of psychrophilic anaerobicdigestion biotechnology for processing ammonia-rich waste

http://dx.doi.org/10.1016/j.apenergy.2014.01.0340306-2619/Crown Copyright � 2014 Published by Elsevier Ltd. All rights reserved

⇑ Corresponding author. Tel.: +1 819 780 7128; fax: +1 819 564 5507.E-mail addresses: daniel.masse@agr.gc.ca (D.I. Massé), rrajinime@yahoo.co.in,

rajinikanth.rajagopal@agr.gc.ca (R. Rajagopal), gursharan351@gmail.com (G. Singh).1 Present address: Biotechnology Branch, UIET, Sec. 25-B, Panjab University,

Chandigarh, India.

Daniel I. Massé, Rajinikanth Rajagopal ⇑, Gursharan Singh 1

Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, 2000 College Street, Sherbrooke, QC J1M 0C8, Canada

h i g h l i g h t s

� Long-term anaerobic digestion (AD) process at high-ammonia (>5 gN/L) is limited.� PADSBR technology was validated to treat N-rich waste with 8.2 ± 0.3 gNH3-N/L.� Excess ammonia (8.2 gN/L) did not affect the digestion process with no inhibition.� VFA, an indicator for process stability, did not accumulate in PADSBR.� Biomass acclimation in PADSBR ensured a high-stabilization of the AD process.

a r t i c l e i n f o

Article history:Received 8 April 2013Received in revised form 27 November 2013Accepted 15 January 2014Available online 8 February 2014

Keywords:Ammonia inhibitionBiomass acclimationPig manurePsychrophilic anaerobic digestionSequencing batch reactor

a b s t r a c t

Ammonia nitrogen plays a critical role in the performance and stability of anaerobic digestion (AD) ofammonia rich wastes like animal manure. Nevertheless, inhibition due to high ammonia remains anacute limitation in AD process. A successful long-term operation of AD process at high ammonia(>5 gN/L) is limited. This study focused on validating technical feasibility of psychrophilic AD in sequenc-ing batch reactor (PADSBR) to treat swine manure spiked with NH4Cl up to 8.2 ± 0.3 gN/L, as a represen-tative of N-rich waste. CODt, CODs, VS removals of 86 ± 3, 82 ± 2 and 73 ± 3% were attained at an OLR of3 gCOD/L.d, respectively. High-ammonia had no effect on methane yields (0.23 ± 0.04 L CH4/gTCODfed)and comparable to that of control reactors, which fed with raw swine manure alone (5.5 gN/L). Longersolids/hydraulic retention times in PADSBRs enhanced biomass acclimation even at high-ammonia. ThusVFA, an indicator for process stability, did not accumulate in PADSBR. Further investigation is essential toestablish the maximum concentrations of TKN and free ammonia that the PADSBR can sustain.

Crown Copyright � 2014 Published by Elsevier Ltd. All rights reserved

1. Introduction considers it essential to promote the good farming practices as a

Hog production is a vital element of Canada’s agricultural econ-omy. The agricultural sector in Québec, one of the largest provincesin Canada, generated 7.5% of total greenhouse gas emissions (GHG)in the year 2006, that is, 6.36 Mt of carbon dioxide equivalents [1];whereas emissions created by pig production, due, among otherthings, to the spreading of swine manure as fertilizer, contributedto about 15% of total farm emissions, which indicates less than 1%of total GHG emissions in Québec [2]. Although the fact that the pigfarming industry is not a leading source of GHG emissions, an asso-ciation is occasionally made between these emissions includingammonia and odours, that is the primary cause for the swine sector

means of reducing these emissions.Replacing fossil fuels with renewable energy, for instance

biomethanization, is an effective manure management options toreduce the total GHG emissions for the agricultural sector [3].Despite these benefits, digestion of swine manure as a sole sub-strate has previously been shown to be unsuccessful, mainly dueto the high content of ammonia in this waste. Ammonia is regu-larly reported as the primary cause of digester failure because ofits direct inhibition of microbial activity [4–6]. Total ammonia con-centration (TAN) greater than 4 gN/L was shown to be inhibitoryduring digestion of livestock manure [4,7,8]. TAN comprises of free(un-ionized) ammonia (NH3) [FAN] and ionized ammonium nitro-gen NHþ4

� �, in which FAN has been suggested as the cause of

inhibition in high ammonia loaded process since it is freely mem-brane-permeable [7,9]. FAN concentration primarily depends onfew important parameters such as TAN, temperature, pH and ionicstrength of the digesting material. An increase in digester

Table 1Properties of swine manure and inoculum.

Parameter Swine manure Inoculum

Total COD (g/L) 146.71 ± 24.5 18.08 ± 2.9Soluble COD (g/L) 42.22 ± 6.0 5.87 ± 0.1

50 D.I. Massé et al. / Applied Energy 120 (2014) 49–55

operating temperature commonly increases the metabolic rate ofthe microorganisms and studies have suggested that increase intemperature will lead to an increase in the fraction of FAN[7,8,10]. Thus, psychrophilic AD process has a tendency to inhibitless and found to be more stable than thermophilic/mesophilictemperatures especially for high-ammonia loaded digesters [11].

Several studies have concentrated on the prevention of variousprocess imbalances, predominantly via development of differentprocess control strategies, automation and augmentation of pro-cess monitoring [11]. Few other studies have attempted to comeup with practical solutions to avoid inhibition and harvest stablebiogas production such as: (i) dilution of reactor content [9,12];(ii) addition of materials – such as bentonite, glauconite and phos-phorite with ion exchange capacity [13,14]; (iii) Struvite precipita-tion [15] and use of carbon fiber textiles [16]; (iv) adjustment ofthe feedstock C/N ratio and pH [11,12]; and (v) lowering tempera-ture from thermophilic (55 �C) to more moderate conditions [40–50 �C] [17]. However, some of these techniques either had asignificant negative effect on methane production or economicallynot feasible; and according to our best understanding none of thesecontrol techniques have been successfully implemented on thefarm scale. Adaptation of microbes especially the methanogens tohigh ammonia concentration could accelerate the ammonia toler-ance [8,18]. Several high-rate anaerobic processes are developedin the recent past and to date, practically all full-scale AD applica-tions are restricted to mesophilic temperatures, which typically re-quire heating for efficient treatment. Dague and co-workers (U.S.Patent No. 5, 185,079) have developed the high-rate anaerobicsequencing batch reactor (ASBR) for the treatment of swinemanure (total chemical oxygen demand (COD), 71.5 g/L; and totalkjeldahl nitrogen (TKN), 4.5 g/L) at psychrophilic (25 �C) tempera-ture. However, to avoid ammonia toxicity in the reactors, they havediluted the raw swine manure by a factor of four. For swine waste,Zhang et al. [19] reported the successful operation of ASBR, withbiogas production rate of 0.9–1.8 L/L/d, at the maximum ammoniaconcentration of 2.5 g/L in the pH range of 6.8–7.4. Zeeman et al.[20] and Wellinger and Sutter [21] have stated that start-up ofanaerobic reactors to be operated at low temperatures needs inoc-ulation with bacteria already acclimated to psychrophilic temper-atures. Besides, the long-term successful operation of the ADprocess at higher ammonia concentrations (i.e. >5 gN/L) has notyet been reported. In addition, such research for the low tempera-ture AD process is limited.

As temperature is considered as a evident factor, which affectsthe threshold of ammonia inhibition, the purpose of this workwas to investigate the technical feasibility of psychrophilic anaer-obic digestion in sequencing batch reactor (PADSBR) to treat swinemanure with excess ammonia nitrogen. We induced the ammoniainhibition by spiking with ammonia chloride (NH4Cl) to labora-tory-scale PADSBRs to simulate the sharp increase in TAN levelsto 8.2 ± 0.3 gN/L that may occurs in actual centralized biogas plantswhen proteinaceous co-substrates are fed to the reactors. This re-search is crucial to prevent ammonia inhibition primarily to getrid of bioreactors failure or avoid reduced performance in termsof energy recovery and biogas plant economic efficiency. Further,the proposed low-temperature AD technology is expected topromote the good farming practices in cold weather countries,for instance North America.

Total solids (%) 10.5 ± 2.0 1.9 ± 0.2Volatile solids (%) 8.7 ± 2.2 1.0 ± 0.1Fixed solids (%) 1.9 ± 0.3 0.9 ± 0.1Total VFA (g/L) 22.1 ± 4.3 0.14 ± 0.01TKN (g/L) 8.4 ± 0.6 5.1 ± 0.4NH3–N (g/L) 6.3 ± 0.4 4.1 ± 0.2pH 6.91 ± 0.2 7.80 ± 0.1Alkalinity (gCaCO3/L) 22.0 ± 2.4 18.1 ± 0.7Phosphorous (g/L) 1.9 ± 0.1 0.58 ± 0.4

2. Methods and materials

2.1. Feedstock and inoculum sources

The fresh raw manure slurry was collected from a manuretransfer tank on a commercial swine operation located in

Sherbrooke, Quebec province of Canada. The manure was screenedto remove particles larger than 3.5 mm to avoid the operationalproblems especially plugging of the influent line with the smallscale digesters. The manure was then mixed to prepare homoge-nize feed samples and stored in a cold room at 4 �C to prevent bio-logical activity. NH4Cl was chosen as the source of ammoniainstead of any other ammonium salt was due to the fact that chlo-ride was reported less inhibitory than ammonium [8]. The inocu-lum was sourced from the farm-scale bioreactor of Peloquin’sswine farm located in Ste-Edwidge de Clifton (Quebec), whichwas already acclimatized to the treatment of swine manure slurry.Average manure and inoculum characteristics during the experi-mental period are given in Table 1.

2.2. Experimental set-up and operating principles of PADSBR process

The anaerobic fermentation of swine manure was performedusing six PADSBRs, such that four identical (replicates) digesterswere used to study the effect of excess ammonia concentrationson the AD process using swine manure spiked with NH4Cl;whereas another two (replicates) PADSBRs were kept as control,which fed with pig manure only. PADSBRs were installed at a con-trolled-temperature room, adjusted at a temperature of24.5 ± 0.5 �C. The sludge volume in the all the reactors were main-tained at 20-L (effective volume, 24 L) and the OLRs were based onthe amount of CODfed (gTCODfed) per L of sludge. All the reactorswere operated for more than 1 year (375 days) and the operatingconditions are presented in Table 2.

A typical operation cycle length consists of 4 weeks which in-cluded the fill, react, settle and draw phases. The fill step involvesthe addition of swine manure to the PADSBR system. The feedingwas carried out on day 0 and 7 of each cycle and the feed volumewas determined on the basis of desired OLR used in this study.During the react phase, the soluble organics and some of the sus-pended organic particulates were transformed into biogas by theanaerobic microorganisms. At the end of every 4 week cycle (i.e.end of react phase), the settling of biomass was completed andthe supernatant (treated) wastewater was drawn out from thePADSBRs leaving 20-L sludge volume before feeding with freshmanure. The volume decanted is normally equal to the volumefed during the fill step. The high food to microorganism (F/M) ratiooccurred immediately after feeding step resulted in high-rate ofsubstrate utilization and hence, high-rate of waste conversion tobiogas. Whereas, towards the end of react phase, the F/M ratiowas at its lowest level with low biogas production, provided idealconditions for biomass settling and thus enhanced longer solids(biomass) retention time. Mixing was done by recirculating thebiogas using a dual-head air pump twice a week for about 5 minjust before taking mixed liquor samples for analysis purpose only.Otherwise, no additional external mixing was employed primarilyto simulate more suitable operational conditions on a commercial

Table 2Operating conditions of the PADSBRs.

No. of replicateASBRs

Substrate Operationtemperature, �C

Sludgevolume, L

OLR,gCOD/L.d

Quantity ofmanure fed, L

Cycle length,week

Fill period React period

4 Pigmanure + addition ofNH4Cl

24.5 ± 0.5 20 3.0 ± 0.35 3.9 ± 1.3a (for onecycle)

4 Day 0 and 7 ofeach cycle

4 and 3 weeks ofeach cycleb

2 (control) Pig manure only

a Fluctuation depends on the manure collected at different periods.b For a 4 week cycle length, the react periods of 4 and 3 weeks corresponds to fill period t = 0 and t = 7 days, respectively.

D.I. Massé et al. / Applied Energy 120 (2014) 49–55 51

farm. This operating strategy was followed for the consecutivecycles of operation. OLR was maintained around 3 gCOD/L.dthroughout the experiment. Daily biogas production was measuredusing wet tip gas meters.

2.3. Ammonia concentration in the digesters

The swine manure used in this study contains ammonia levelsmerely in the range 5–6 gN/L. Hence, four PADSBRs (R1–R4) werespiked with NH4Cl together with the addition of swine manureto study the effects of high ammonia concentration in the dige-state; whereas, reactors R5 and R6 were kept as control digesterstreating swine manure without the addition of excess ammonianitrogen. The total ammonia concentrations in the reactors R1–R4 were increased to a value of 8.2 ± 0.3 gN/L compared to5.5 ± 0.7 gN/L for the control reactors (R5–R6).

2.4. Sampling and analytical procedures

A mixed liquor samples of about 100 mL capacity was takenbiweekly from the PADSBRs after 5 min of mixing by recirculatingthe biogas. At the end of each cycle (i.e. 4 weeks), the settled bio-mass and the supernatant (treated) effluent were also collectedfor their physico-chemical characteristic analysis. Raw swinemanure was sampled during the fill period. These samples wereanalyzed for total COD (TCOD), soluble COD (SCOD), total solids(TS), volatile solids (VS), volatile fatty acids (VFAs) such as acetic,propionic, butyric, iso-butyric, valeric, iso-valeric and caproic, pH,alkalinity, TKN and ammonia nitrogen.

The pH value was measured immediately upon collection ofsamples using PH meter (model, TIM840, France). TCOD and SCODwere determined by the closed reflux colorimetric method [22].SCOD of fresh manure and effluent samples was determined byanalyzing the supernatant of slurry samples after centrifugation.VFAs concentration was determined using a Perkin Elmer gas chro-matograph model 8310 (Perkin Elmer, Waltham, MA), mountedwith a DB-FFAP high resolution column. Before VFAs quantifica-tion, samples were conditioned according to the procedures de-scribed by Massé et al. [23]. Alkalinity, TS and VS weredetermined using standard methods [22]. TKN and NH4–N wereanalyzed using a Kjeltec auto-analyzer model TECATOR 1030(Tecator AB, Hoganas, Sweden) according to the macro-Kjeldahlmethod [22]. Daily biogas production was measured using wettip gas meters. Every week, biogas composition (methane, carbondioxide, and nitrogen) was determined with a HachCarle400 AGC gas chromatograph (Hach, Loveland, CO). The columnand thermal conductivity detector were operated at 80 �C. Thenitrogen content was subtracted from the results, because N2 gaswas used as a filler gas during drawdown.

2.5. Calculations

The free ammonia level was calculated according to Koster [24].It was reported that the fraction of free ammonia relative to the

TAN is dependent on pH and temperature, as reported in Eqs. (1)and (2). The percentage of free ammonia to that of total ammoniaconcentration was determined using Eq. (3)

NH3ðFreeÞ ¼ TAN1

1þ 10�ðpKa�pHÞ

� �ð1Þ

pKa ¼ 0:09018þ 2729:92T

� �ð2Þ

NH3;% ¼½NH3� � 100½NH3� þ ½NH4þ �

ð3Þ

NH3 is the Free ammonia nitrogen (FAN), mg/L; NHþ4 is Ammoniumion, mg/L; TAN is Total ammonia nitrogen, mg/L; pKa is Equilibriumionization constant; and T (K) is the Temperature (Kelvin).

3. Results and discussions

Four laboratory-scale PADSBRs spiked with concentratedammonia were monitored for more than a year to assess theirreliability and stability in terms of organic matter removal, VFAelimination and biogas production. The average OLR applied tothe bioreactors was in the range of 3 gCOD/L.d, with a TCOD con-centration in the feed around 146.7 gO2/L. The pH of raw manurewas about 6.91 (near neutrality), although high VFA concentrationsof 22.1 g/L were detected, mostly because of the high amount ofalkalinity (�22 gCaCO3/L) in the manure.

3.1. Effect of excess ammonia N on organic fractions removal andmethane production

The PADSBRs (R1–R4) and control reactors (R5–R6) were oper-ated in parallel under similar operating conditions as presented inTable 2. The summary of the results obtained for the removal oforganics such as TCOD, SCOD, TS and VS in the treated liquor alongwith methane production is given in Table 3 and an illustration ofthe profile for the cumulative methane production is presented inFig. 1.

Similar profiles were attained for the PADSBRs pulsed with NH4-

Cl to that of control reactors with regard to COD removal efficien-cies, cumulative methane production and methane yield. Althoughthe difference is minimal (i.e. about 5% variance) after 200 days ofoperation, a comparatively lower methane production was re-corded for PADSBR spiked with NH4Cl to that of control digesters,which probably explained by the higher ammonia concentrations.Whereas, solids removals were relatively higher in the controlreactors (Table 3). The probable reason could be that in PADSBRs,organic matter is reduced by biological conversion into methaneand by physical removal during the settling period [25]. Since thereis no significant differences observed in the methane productionfor all the reactors, differences in solids reduction may be due tothe variances in physical removal and likely effect of NH4Cl saltused as a source of ammonia nitrogen in PADSBRs R1–R4. The com-position of biogas with methane content of 68–70% showed that

Table 3Removal of organic fractions and methane production.

Reactors Period of operation,days

OLR, gCOD/L.d

Total CODfed percycle, g

Reduction efficiency, % removal Methane yield, L CH4/gTCODfed

aCH4 content ofbiogas, %

TCOD SCOD TS VS

R1–R4 375 3.0 ± 0.35 504 ± 105 86 ± 3 82 ± 2 67 ± 4 73 ± 3 0.23 ± 0.04 68.3 ± 2.4(0.48 ± 0.09)b

R5–R6(control)

88 ± 1 84 ± 2 77 ± 4 84 ± 3 0.24 ± 0.05 70.2 ± 2.9(0.49 ± 0.10)b

a Values corresponding to the last 5 cycles.b Values in parenthesis (�) indicate methane yield based on VS loading (L CH4/g VSfed).

0

40

80

120

160

200

0 50 100 150 200 250 300 350 400

Met

hane

(m

L)

Time (days)

PADSBR (R1-R4)

Control Reactor (R5-R6)

Fig. 1. Cumulative methane production.

52 D.I. Massé et al. / Applied Energy 120 (2014) 49–55

the biogas obtained was of good quality. Even if the pH was notcontrolled in the bioreactors there was no formation of foam andscum observed during this study period. The mode of operation(process, temperature) and the appropriate choice of acclimatizedinoculum at the start-up of experiment allowed a high-stabiliza-tion of pig manure digestion even at high total ammonia concen-trations (8.2 ± 0.3 gN/L).

From Fig. 1, it is shown that the cumulative methane productionfor the fourth cycle of operation, i.e. day 59–94, was comparativelyhigher. As this 4th cycle was performed in the month of Christmas(December), the cycle length was increased to 35 d in contrast to28 d for other cycles. To compensate longer retention time, we in-creased the amount of total COD fed particularly for this cycle with830 g compared to that of other cycle, which was maintained in therange 504 g of TCOD. In addition, fluctuations in COD value weredue to change in manure characteristics. However, relatively high-er values for the cumulative methane production after day 275(Fig. 1) than the initial periods showed that the active biomassaccumulated in the settled sludge enriched the performance ofPADSBRs with time. Effective sedimentation occurred in the PADS-BRs, which reduced substantially the biomass washout in the efflu-ent. Thus, for the studied OLR (i.e. 3 COD/L.d), the addition ofexcess ammonia nitrogen to the pig manure did not affect thestability and performance of the PADSBRs.

3.2. Effect of excess ammonia N on VFA accumulations

AD instability can happen due to the accumulation of VFA con-centrations with a concurrent decrease in methane gas production.Hence, the fate of different components of VFA was followedprimarily to investigate the possibility of methanogens inhibition.

Fig. 2(A and B) illustrates the pH and the typical profiles of shortchain fatty acids (SCFAs) such as acetic (C2), propionic (C3), butyric(C4), iso-butyric (iC4), valeric (C5), iso-valeric (iC5) and caproic (C6)

during one cycle of operation (4 weeks) for the representativePADSBRs (in the mixed liquor) with and without addition of excessammonia. Similar VFA dynamics were observed in all the digestersbut with different values. Acetic acid was the predominant VFAcomponent produced during the digestion of pig manure, whichcomprised more than 73% and 85% of the total VFAs for the PADS-BRs (with excess ammonia addition) and the control reactors,respectively. Whereas, propionic acid contained about 15% and7% of the total VFAs produced, respectively and the higher molec-ular weight VFAs (C4–C6) were produced in negligible amounts(Fig. 2). As expected, higher VFA concentrations were observed justafter the time of feeding (i.e. on day 0 and 7) due to the hydrolysisof complex molecules and acidogenesis, and also partly due to thehigh VFA concentrations in the swine manure fed to the bioreac-tors, as indicated in Fig. 2. Total VFAs produced (maximum of3235 mg/L) in the beginning of a 4 week cycle were eliminated to-wards the end (VFA < 100 mg/L), showed that VFAs did not accu-mulate in the PADSBR even at higher ammonia N concentrations.Acclimatized methanogens could able to consume most of theSCFAs produced within 15–18 days. Swine manure is a highly buf-fered waste and hence alkalinities in all the digesters were found tobe optimal with an average value of 25,058 ± 2634 and26,322 ± 2701 mg CaCO3/L for the PADSBRs (R1–R4) and controldigesters (R5–R6), respectively. A small deviation of less than onepH unit during cycles was observed as shown in Fig. 2, which couldbe explained by the high buffering capacity of swine manure.

Lauterböck et al. [26] observed the accumulation of VFA, espe-cially propionic acid, as well as the decline of biogas productionwhile digesting slaughterhouse waste, particularly when the TANconcentration exceeds 6 gNH4-N/L at 38 �C and pH of 8.1. For a cat-tle manure digestion in a CSTR, Angelidaki and Ahring [17] wit-nessed that high ammonia concentration (TAN levels up to 6 gN/L) inhibited the methane production at thermophilic temperatures(55 and 64 �C) and resulted in a rapid increase in VFA concentra-tions (5000 mg/L) at pH 7.9. Similar results were observed usingthermophilic UASB reactors by Borja et al. [27], in which the VFAconcentrations increased from 1000 to 3000 mg/L as acetic acidwith increase in ammonia concentrations up to 7 gN/L. Whenswine manure was anaerobically digested at temperatures from37 to 60 �C, the amount of VFA increased with increasing temper-ature from 4800 to 15,800 mg-acetate/L [5]. In the PADSBRs, theVFAs, an indicator for the process stability, was much lower anda higher gas yield associated with enhanced degradation was ob-served in the present study. The psychrophilic SBR approach offersan attractive know-how to improve the process efficiency in AD ofammonia rich wastes with TAN levels of 8.2 g/L.

3.3. NH4–N, free ammonia and TKN concentrations

An increment of total ammonia levels NH3 þNHþ4� �

wasobserved in all the reactors such that average initial TAN concen-trations augmented from 7.9 and 5.0 g/L to 8.3 and 6.3 g/L for thePADSBRs (R1–R4) and controls (R5–R6), respectively. This increase

D.I. Massé et al. / Applied Energy 120 (2014) 49–55 53

was probably due to (i) conversion of some organic nitrogen(mainly protein and urea) to ammonia during AD [28]; and (ii)accumulation of NH4–N as more manure was fed to the bioreactors[29]. Similar profiles were observed for the TKN concentrationswith average values significantly increased from 8.9 and 5.7 g/Lto 9.7 and 7.8 g/L for the PADSBRs and controls, respectively.

It is likely that inhibition by ammonia in the AD process shouldalso be related to the FAN concentrations rather than TAN orammonium ions, as it is considered to be the foremost reason forinhibition of methane-producing consortia [12,18]. Average FANconcentrations in the PADSBRs and control digesters were ob-served in the range of 184 and 147 mg/L, respectively. FAN concen-tration was calculated by using ionization equation (Eqs. (1) and(2)) and taking pKa of 9.26 for 24.5 �C (digester temperature). Thecontrol digesters showed relatively lower FAN levels than PADS-BRs, however, in our study ammonia levels were significantly low-er than the threshold concentrations reported in previousinhibition works [5,18]. In the present study, free ammonia levelscontained about 2.27% and 2.61% of TAN, i.e. sum of NH3–N andNHþ4 —N concentrations. Furthermore, some studies have shownthat high levels of free ammonia has been proven to cause accumu-lation of VFA components, indicate an imbalanced microbiologicalactivity and propionate degradation when the total ammonia con-centration is around 4.0–5.7 g/L [30,31]; but again in the presentstudy the excess total ammonia N did not affect the PADSBR pro-cess. However, to establish the maximum concentrations of TKNand free ammonia that the PADSBR can sustain, further detailedinvestigations are in progress.

3.4. Concluding remarks: potential inhibitory compound (ammonia) inAD

Typically, swine manures contain approximately 4–6 gN/L onaverage. For swine manure, AD inhibition by ammonia reportedto occur at the TAN concentrations of 1.5–2.5 g/L [5,32]; whereas,a wide range of inhibiting ammonia concentrations were reported(1.5–7.0 g/L) for livestock wastes including animal by-products[6,33]. The wide range of values are probably due to the differencesin nature of substrates, inocula, environmental conditions (tem-perature, pH) and acclimation periods [4]. In particular, McCarty[33] reported that when ammonia nitrogen concentration exceeds3 gNH4-N/L, the AD processes are inhibited at any pH. Whereas,Hobson and Shaw [34] showed that TAN concentration of2.5 gNH4-N/L resulted in some inhibition of methane production,while a concentration of 3.3 gNH4-N/L inhibited methanogenesiscompletely. For an adapted process, Angelidaki and Ahring [7]specified that a TAN tolerance of up to 3–4 gNH4-N/L, while treat-ing cattle manure under thermophilic conditions. These results arein agreement with the studies reported by Sung and Liu [8] andProcházka et al. [10], where they have demonstrated that higherTAN concentrations (>4.0 g/L) could cause obvious inhibition ofmethanogenesis for the thromophilic (55 �C) and mesophilic (35–40 �C) temperatures, respectively. The low temperature digestionprocess presented in this work shown to have a stable performanceeven at TAN level of 8.2 g/L, which corresponded to an OLR of3 gCOD/L.d. In addition, PADSBR technology has the advantage ofworking with more robust microorganisms that due to their lipidrich structure, as previously described by Russell [35], can copewith high ammonia content in the waste that most anaerobic sys-tems would be sensitive to.

Our previous work [29] revealed that the psychrophilic anaero-bic digestion was not affected by ammonia and VFA concentrationsof about 4 gN/L and 8.7 g/L, respectively and by variation in oper-ating temperatures process (20–10 �C) and OLRs (1.2–1.6 gCOD/L.d). In the present study, although we maintained the similaroperating strategies, PADSBR was performed under higher TAN lev-

els of about 8.2 g/L and at an OLR of about 3 gCOD/L.d, which is al-most two times higher than our previous work. The novelty of thiswork was relied on the adaptation of microbes especially themethanogens to high ammonia concentrations, which acceleratedthe ammonia tolerance. Step-wise increase of ammonia concentra-tion could be enhanced the adaptation of the cells gradually over along period. Acclimated microbial communities thus consumedsubstrate without lag and intermediate substrates did notaccumulate.

An inhibitive threshold of 1.1 g/L of FAN levels was reportedby Hansen et al. [5] for mesophilic and thermophilic conditionswith biomass adapted to high ammonia concentrations over along period. In a similar study, Nielsen and Angelidaki [9] de-scribed that FAN concentration of 1.2 gN/L inhibited the digestionof cattle manure at pH 7.6 at 55 �C. Besides, such research for thelow temperature AD process has been scarce. Hence, this researchis crucial to obtain the inhibitory limits for the psychrophilic ADtechnology, which is expected to promote the good farming prac-tices especially suitable for the cold weather countries like NorthAmerica.

Despite some advantages for the psychrophilic operating con-ditions, a drop in digester operating temperature could consider-ably decelerate the microbial activity and disturb the treatmentefficiency [29]. This is associated with the biomass loss in termsof mixed liquor VSS concentrations that could disrupt the PADS-BR process by affecting biomass retention. Thus, to ensure thesteady and efficient degradation of organics at high ammonialevels, adequate biomass settling is prerequisite for the PADSBRsystems. Accordingly, in the present work, long term biomassadaptation was carried out to investigate the inhibitory effectsof ammonia nitrogen on AD by controlling factors such as tem-perature, biomass retention time and OLR, while pH was not ad-justed. PADSBRs, ensured an effective sedimentation, thus thisprocess immensely minimized the washout of active solids inthe treated effluent.

In summary, a precise limit for the ammonia concentrations,which should not be exceeded, is hard to fix as the toxic effectdepends hugely on the substrate concentrations, relevant fermen-tation conditions (pH, temperature), available bacterial strains andacclimatization period. However, a successful operation of AD re-ported in the present study shows that methanogens in PADSBRare capable of adaption to higher concentration of ammonia(8.2 gN/L) at 24.5 �C and a pH of around 7.8. The longer solidsand hydraulic retention times in PADSBRs enhanced the biomassacclimation at these reported TAN levels. Organic matter and VFAconcentrations were relatively lower in the treated effluent, illus-trating that the performance of PADSBR was stable and efficientthroughout the study period. PADSBR has proved to be a lessenergy intensive technology and would certainly be an attractiveoption for the farms, as the requirements for the reactor mixingand heating is considerably fewer.

The fraction of free (undissolved) ammonia has been reportedto increase with temperature and pH [8], which is commonly be-lieved to be the actual toxic agent than ammonium ions as it iscapable to penetrate through the cell membrane. In this study,without pH adjustments of the digested pig slurry (pH 7.8), degra-dation of propionate, butyrate and valerate (Fig. 2) as well as meth-ane production (Fig. 1) were still feasible regardless of its high TANconcentration of 8.2 gN/L. The lower final acetate and propionateconcentrations indicated that the acetoclastic methanogens andthe syntrophic propionate-degrading acetogenic bacteria-hydro-genotrophic microorganisms were not inhibited at the reportedammonia concentration (TAN: 8.2 g/L; FAN: 184 mg/L) and pH of7.8. As FAN levels are considered to be toxic agent for methano-gens, further investigation is under way to obtain the inhibitivethreshold level of free ammonia in PADSBR.

Fig. 2. Acetic (C2), propionic (C3), butyric (C4), iso-butyric (iC4), valeric (C5), iso-valeric (iC5), caproic (C6) and pH in the mixed liquor during one cycle (A) PADSBRs with TANlevels of 8.2 ± 0.3 g/L; (B) control reactor with TAN levels of 5.5 ± 0.7 g/L.

54 D.I. Massé et al. / Applied Energy 120 (2014) 49–55

3.5. Benefits of PADSBR technology

Properly functioning PADSBR system is expected to yield a widerange of benefits for their users and the environment in general.The foremost profits include the production of energy in terms ofheat, light or electricity. For reference, a growing-finishing pig pro-duces an average of 7.6 L of manure per day [36]. Considering thespillage from waterers, floor washing and dilution water, the liquidmanure from a storage tank accounted to a minimum of 21.6 m3 ofliquid manure produced per sow annually. In this study, the man-ure obtained from the local pig farms has average contents of146.7 gCOD/L (Table 1), which means that a pig can produce about3169 kg of COD annually. Using the methane yield values given inTable 3, a growing-finishing pig can produce about 728 m3 of CH4

annually. In Quebec province alone the hog production was esti-mated over 7.2 million [37] and as the calorific value of biogas isabout 6 kW h/m3, which corresponds to 0.5 L of diesel oil [38], thisPADSBR technology would lead to saving of an enormous amountof fuel per year. Alternatively, biogas can be converted to electricityusing a biogas powered electric generator and in such circum-stances, we get about 2 kW h of useable electricity per cubic meterof biogas, the rest turns into heat which can also be used for heat-

ing applications. By extracting CH4 out of waste and using it to har-vest heat or electricity also reduces the direct methaneatmospheric emissions and thus GHG impact.

Apart from this, due to the presence of more stable microbes,PADSBR technology can anaerobically digest wastes with highammonia content (up to 8.2 gN/L) and possibly transforms the or-ganic wastes into high quality fertilisers. As PADSBR operates be-low 20–25 �C, no additional energy is required to heat thedigester as in the case of mesophilic and thermophilic processes;and waste heat can be gleaned from the engine and there couldbe still excess heat available throughout the year. The other bene-fits include: improvement of hygienic conditions through thereduction of pathogens; micro- and macro-economical benefitsthrough energy and fertiliser replacement and decentralized en-ergy generations.

4. Conclusions

This study demonstrated the robustness of PADSBR technologythat can be employed to limit ammonia inhibition even at higherconcentrations. Higher TAN levels up to 8.2 gN/L did not affect

D.I. Massé et al. / Applied Energy 120 (2014) 49–55 55

the anaerobic digestion of pig manure. The mode of operation (pro-cess, temperature) along with the acclimation of biomass at higherammonia concentrations ensured a high-stabilization of the diges-tion process without inhibition and thus VFA components did notaccumulate in the digester. The possible inhibition by FAN wasinsignificant for the studied period and an extensive research isnecessary to obtain the inhibitive threshold level of FAN inPADSBR.

Acknowledgements

This project has been financially supported by contributions ofAgriculture and Agri-Food Canada and Bio-Terre Systems Inc.

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