Article pubs.acs.org/IECR
The Zootechnical Anaerobic Supernatants: Nutrient Removal by a Biological Advanced Process M. Santinelli,† A. L. Eusebi,†,* M. Santini,† and P. Battistoni† †
Department SIMAU, Università Politecnica delle Marche, Via Brecce Bianche, 60100 Ancona, Italy ABSTRACT: This paper deals with the evaluation of total nitrogen (TN) removal from zootechnical wastes in a pilot scale reactor, treating digested supernatants by biological way. The high performances in terms of total nitrogen removal by nitrites, greater than 95%, enabled to design a medium scale treatment platform, by combining the anaerobic digestion (AD) and dewatering (DW) units with the alternate cycles (AC) biological process for pig and cattle supernatants. Theoretical mass balances carried out on maximum loading conditions allowed to define an optimal process chain and its sizing. Influent and effluent main parameters were determined for each operation unit, defining a final TN value for pig slurry equal to 14.1 g d−1 head−1 in the dewatered amount and 12.3 g d−1 head−1 from the AC reactor, besides an inner TN load of 49.6 g d−1 head−1. Concerning the cattle slurry treatment, the DW process gained a TN removal of 74%, allowing the AC process to remove the remaining amount of TN of about 30.1 g d−1 head−1. An evaluation of the reliability of a fermentation process to support the anoxic phase of the biological process was further investigated.
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INTRODUCTION The requirement to preserve the aquatic systems from eutrophication phenomena has become, in last decades, an essential goal regarding the control of pollution of anthropic origin. Particularly, the wide increase of livestock farming in the last years determined the need to preserve the quality of waters from nutrients, to reduce the contamination. The diffuse practice of spreading the manure dejections in agricultural areas has been estimated by the European Energy Agency as the most significant factor of incoming nutrients in water bodies, with a nitrogen release from intensive systems of about 60%.1 Dejections from livestock breeding contain themselves elevated concentrations of nitrogen and phosphorus, which increase when treated by the anaerobic digestion process. The requisite to reduce the nitrogen load from livestock dejections led us to consider and examine various treatment processes, in order to define optimal operative conditions and to meet the more restrictive law limits, defined as 170 kg TN ha−1 year−1 to spread in nitrates vulnerable areas.2 A complete nitrogen removal could be achieved preventing the complete oxidation of ammonium, retaining the reaction into nitrite formation. The further step must be the denitritation of nitrite to nitrogen gas. This nitrite pathway may yield up to a 25% reduction in aeration and 40% reduction in COD requirements. Many examples are present in the scientific literature to create the environmental conditions to enhance the nitritation process.3−7 As reported in Table 1, the microaerobic processes, studied in laboratory scale, (DO lower than 2 mg L−1) determined an ammonia removal from 63% up to 100%, independently from the reactor configuration adopted. Moreover, in the last years, some fully functional nitritation processes, as SHARON and Anammox processes,8,9 have been developed in full scale wastewater treatment plants. The SHARON process is based on partial ammonium oxidation operated by autotrophic bacteria followed by reduction in anoxic environment, while Anammox process consist in a partial ammonium oxidation © 2012 American Chemical Society
Table 1. Ammonia Performances Comparison in Different Microaerobic Processes process
DO, mg L−1
E % NH4, %
ref
activated sludge biofilm CSTR moving bed biofilm activated sludge SBR
0.7 0.5 2.0−4.0 3.0
67 100 50 88 50
Ruiz et al., 20033 Bernet et al., 20014 Fux et al., 20025 Chung et al., 20076 Gali et al., 20077
operated by a different kind of autotrophic bacteria (Anammox bacteria), that are able to oxidize ammonium using nitrite as electron acceptor. The main performances obtained as ammonia reduction were defined up to 95%.10 The aim of this study is to investigate the performances obtainable with a continuous alternate oxic and anoxic biological process (AC) to reduce the nutrients loads from the supernatants of swine and cattle flows anaerobically digested. The validation of the biological process was made in two different sizes. First, a bench experimental reactor, fed with zootechnical and civil wastes anaerobic digested, performed the AC biological process to determine the optimal operative conditions and the obtainable performances. Next, a specific nitrogen mass balance to size a complete flow scheme (anaerobic digestion unit and biological reactor) was conducted. The results were utilized to design a medium size platform to validate the nitrogen supernatants removal by biological way.
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MATERIALS AND METHODS Continuous Alternate Biological Process. The alternate process disposed for the biological pilot plant unit and designed for the medium size platform is characterized by a continuous
Received: Revised: Accepted: Published: 5490
November 21, 2011 March 21, 2012 March 26, 2012 March 26, 2012 dx.doi.org/10.1021/ie202691n | Ind. Eng. Chem. Res. 2012, 51, 5490−5496
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Figure 1. Pilot scale plant.
Table 2. Operative Conditions of the Bench Scale Reactor Qin, L d−1
CivSup, %
ZooSup, %
18
67
33
Q
methanol,
L d−1
2
HRT, d
aerobic length, min
anoxic length, min
MLSS, mg L−1
MLVSS/MLSS, %
3.5
90
90
8908
81
Figure 2. Unit operations of the experimental platform.
the AC reactor, provided with one blower and mixer (Figure 1). The denitrification phase is supported with the dosage of rapidly biodegradable carbon, automatically added as methanol. The effluent tank is characterized by a level control to permit the collection, each day, of an average sample before the discharge. The influent, effluent, and activated sludge were analyzed according to Standards Methods13 to determine the main physic-chemical parameters and macropollutants. Moreover, the determination of nitrification and denitrification rates (kn and kd) was run on the basis of Kristensen procedure.14 After a first phase of start up for the growth of the biomass in the reactor fed with only civil anaerobic digested supernatants (CivSup), the reactor was supplied by an influent (ZooSup) formed by cattle slurry (33%) and civil supernatant (67%), in accordance with the operative conditions reported in Table 2. The global HRT of the system (influent and methanol flows) was imposed equal to 3.5 d, with maximum lengths of aerobic and anoxic phases of 90 min, modulated by the automatically control device on the basis of the nitrogen influent load. The average DO concentration in the reactor was maintained equal to 1.7 mg L−1. The rapidly biodegradable carbon source was dosed as methanol during the denitrification phase to ensure no limiting condition for the nitrites and nitrates reduction. The mixed liquor concentration was maintained on average of about
influent flow and subjected to oxic and anoxic phases automatically regulated. According to the AC process, the durations of the alternating oxic−anoxic phases were automatically determined the bending points (optimal condition) processing the online signals of dissolved oxygen (DO) and oxidation reduction potential (ORP) probes.11 The percentages of nitrogen forms reduction are absolutely related with the process control automation and the phases lengths are controlled on the basis of imposed minimum and maximum oxic and anoxic times. Statistical software was purposely engineered to evaluate the reliability of the control system and to optimize the process performances.12 The software defines the time lengths of the aerobic and anoxic phases, alternated into the activated sludge tank, and the end-reason that led the automatic control device to switch from aerobic to anoxic phase and vice versa (set point maximum time, maximum DO and/or ORP, optimal condition). Pilot Scale Reactor and the Operative Conditions. The bench scale pilot is formed for 30 days after the start up by an influent stirrer tank, a biological reactor (total volume 70 L) and a secondary clarifier (total volume 30 L). The biological unit is characterized by the DO and ORP probes to perform the alternate cycles and by pH and TSS probes to completely monitor the process. A mixture from civil and cattle anaerobic digested supernatants slurry is sent from the influent tank to 5491
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Table 3. Influent and Effluent Characterization influent pilot reactor characterization NOx−N, mg L−1
NH4−N, mg L−1
pH
alkalinity, mg L−1
COD, mg L−1
TSS, mg L−1
8.01 ± 0.12
5939 ± 1468
6087 ± 1877
2826 ± 1570 3 ± 2.2 1317 ± 285 1688 ± 367 effluent pilot reactor characterization
pH
alkalinity, mg L−1
8.5 ± 0.18
774 ± 283
COD, mg L−1 TSS, mg L−1 2420 ± 480
175 ± 117
TKN, mg L−1
TN, mg L−1
COD/TN
PO4−P, mg L−1
1693 ± 341
3.55 ± 0.50
101 ± 35.5
NO2−N, mg L−1
NO3−N, mg L−1
NH4−N, mg L−1
TKN, mg L−1
TN, mg L−1
PO4−P, mg L−1
0.8 ± 1.9
2.0 ± 3.2
61.6 ± 42.9
89.6 ± 2.8
92.4 ± 73.7
17.8 ± 6.5
8908 mg L−1 with a MLVSS/MLSS percentage of about 81% (Table 2). Operative Units of the Designed Experimental Platform. The designed platform will be located on an area of about 150 m2 inside a full scale wastewater treatment plant. In order to obtain removal performances of nitrogen, on the average equal to 50%, on the zootechnical dejections, the process chain forecasts the anaerobic digestion (R1-AD) and dewatering (DW) units, coupled with the alternate cycles (R2AC) biological process, followed by the sedimentation phase (S7) (Figure 2). Many intermeddle storage tanks (S-1, S-2, S-3, S-4, S5, S6, and S8 with respectively specific volumes of 3 m3, 1.2 m3, 1.0 m3, 3.0 m3, 1.0 m3, 5.0 m3, and 3.0 m3; Figure 2) were located to ensure an operative and continuous autonomy of the platform units of about 30 days. During the design phase, the specific mass balances of each unit and the global one permitted to determine the nitrogen removal and the flows characterization. Concerning the zootechnical dejections transported from the farms with 4 tanks (1 m3 of volume for each; C-1/4 of Figure 2), the degradation of the organic compounds due to the AD process (Figure 2) is performed in a closed reactor of 1.2 m3 of volume (R-1), furnished with a mixer and a couple of resistances in diathermic oil (3000 W). The system is provided with a monitoring system of the internal temperature, able to regulate the thermal flow. The next dewatering process (DW of Figure 2) (2.5 m3 h−1, 2500 rpm) will permit to treat the effluent supernatant by the biological alternate cycles process, while the solid amount will be investigated to determine its environmental impacts related to agronomic reutilization or disposal. Referring to the sludge line, the excess sludge from the AC unit could be directly dewatered or, before, redigested to ensure an adequate hydraulic retention time to the AD compartment (Figure 2). The biological reactor (R-2) (volume of 2.8 m3) performing the AC process15 is constituted by three sequential tanks flowed by gravity, provided with one blower and mixer in each tank to alternate oxic and anoxic phases, the variation of which will be automatically regulated on the basis of signals of dissolved oxygen (DO) and oxidation reduction potential (ORP) probes. The effluent after a secondary sedimentation phase (S-7 of Figure 2) will be monitored, taking average samples in the final S-9 reactor (Figure 2) before the discharge. Moreover, the installation of TSS probes in the main biological reactor, recycler, and waste flows was planned. All the main flows will be measured as amount (online flowmeters) and as chemical and physical characterization, in accordance with Standard Methods.13
L−1) resulted in total suspended solids lower than literature values, 16 probably on account of a previous original sedimentation phase at the farm where the zootechnical digested was collected. The ammonia and TKN concentrations, in line with published data, of about 1317 ± 285 mg L−1 and 1688 ± 367 mg L−1, defined a value of organic nitrogen equal to 371 mg L−1, with the near absence of nitrates and nitrites. By considering the low ratio between rapidly biodegradable carbon and total nitrogen, on the average equal to 0.35, the dosage of methanol was required to maintain the COD/TN ratio of about 3.55 ± 0.5. The biomass inside the reactor reached a medium concentration of about 9048 mg L−1, while the MLSS/ MLVSS ratio was approximately equal to 0.81 during the whole period. The nitrogen loading rate was maintained within the range from 0.20 kg TN m−3d−1 to 0.55 kg TN m−3 d−1, with an average value equal to NLR 0.45 kg TN m−3 d−1. The process reliability defined removal efficiencies of total nitrogen and ammonia of about 95 ± 3.2% and 95 ± 2.4%, respectively. The average effluent concentration of TN was equal to 92.4 ± 73.7 mg L−1, with an NH4−N amount of 61.6 ± 42.9 mg L−1. High treatment performances were reached even for carbon, solids, and phosphorus: removal percentages of 69%, 94%, and 82% were gained, with medium effluent concentration of 2420 ± 480 mg COD L−1, 175 ± 117 mg TSS L−1, and 17.8 ± 6.5 mg PO4−P L1− (Table 3). The effluent PO4−P concentration was, on the average, equal to 17.8 mg L−1, and by considering the pH value inside the tank, it would be interesting to investigate the phosphorus precipitation occurrence. To understand the nitrogen removal behaviors, batch scale tests were conducted on 1 L of the biomass of the pilot scale reactor to determine the ammonia and nitrogen uptake rate (AUR and NUR) values and to demonstrate the reliability of the process to individuate the reduction in NH4−N and NOx−N concentrations (Figure 3). The AUR batch test was performed in not-limiting DO condition (3 mg L−1) and showed that the ammonia concentration from 143 mg L−1 reached values of about 46.3 mg L−1 (Figure 3). Moreover, the global ammonia trans-
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RESULTS AND DISCUSSION Bench Scale Results. A characterization of the mixture used to feed the pilot scale reactor included the methanol source is reported in Table 3. The average concentration of COD (6087 ± 1877 mg L−1) and of TSS (2826 ± 1570 mg
Figure 3. Bench scale test: aerobic−anoxic cycle. 5492
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oxygen demand, pH, and alkalinity values. Concerning the swine dejection, the characterizations of agricultural compounds18 and dewatered pig slurry19,20 were considered to best determine the composition of the sewage. Referring to manure as sampled, a value of TS equal to 90 g L−121 has been chosen, while the TSS value was considered equal to 6%, with a percentages of TVS/TS of 70%, on the basis of the available data. The TN was assumed equal to the TKN because of the near absence of nitrates and nitrites and the soluble nitrogen TNs equivalent to the NH4−N amount, while the remaining fraction was evaluated by the particulate nitrogen TNp. Hence, the ratio between NH4−N and TKN was supposed TNs/TN equal to 0.75. Moreover, the COD/TVS and CODp/TVS were chosen equal to 1.8 and 1.2, respectively. Concerning the TP and alkalinity values, the parameters were assumed equivalent to 128 g d−1 and 28 g d−1, in that order.14 The cattle manure is generally characterized by a higher content of solid fraction, due to the applied stabling, hence the TS values has been considered of 16%,22 with a TSS amount of 120 g/L, equal to 12%,16 as the TVS one. This assumption enabled to extract the COD/TVS ratio of 1.36, while the nitrogen partition, counted on the same hypothesis of the swine slurry, permitted to determine a NH4−N/TKN part of 0.21.22 Design Phase of the Demonstrative Platform: The Mass Balances. The sizing of the experimental platform is based on the operative requirement of the anaerobic digestion process (volume of 1.2 m3), according to the organic loading rate (OLR) parameter, which value varies between 2.5 and 5 kg TVS m −3 d−1 or 4.4 and 8.8 kg COD m3 d−1. Hence, on the basis of the influent flow rates, it was possible to define the minimum (5 swine and 0.5 bovine species) and the maximum (10 and 1 heads of swine and cattle) loading conditions (Table 4). Particularly, to design the process units, this last setting permitted to determine the influent characterization, by defining influent flow rates equal to 97 L d−1 and 50 L d−1 for swine and bovine dejections. Both the matrixes were defined by similar values of pH (of 8.9 and 8.3 for cattle and swine slurry), while a substantial difference in alkalinity was noticed: the bovine manure was characterized by higher values, of about 996 g d−1. Moreover, the soluble amount of nitrogen in the pig slurry (372 g d−1 rather than 24 g d−1 of the cattle slurry) determined a net disparity between TKN concentrations (Figure 4). Regarding the solids partition, both the slurries
formation was mainly related with nitrite increment. In fact, for 96.8 mg NH4−N L−1 oxidized, the total final concentration of NOx−N produced was 83.8 mg NOx−N L−1 formed by 76.9 mg L−1 of nitrites and 6.8 mg L−1 of nitrates. Also, the following anoxic NUR test, obtained adding methanol source, defined as denitritation contribution is the main mechanism of nitrites and nitrates transformation. The 83.8 mg NOx−N L−1 produced from the previous aerobic condition are reduced up to 71.1 mg NOx−N L−1 at the end of the NUR phase. The 21.9 mg NOx− N L−1 reduced are composed for 20.1 mg L−1 by nitrites (Figure 3). The test enabled to highlight maximum nitrification and denitrification kinetic coefficients of 0.18 kg NH4−N kg MLVSS−1 d−1 and of 0.16 kg NOx−N kg MLVSS−1 d−1. Design Phase of the Demonstrative Platform: The Influent Characterization. From the data literature could be possible to determine the main influent characterization for the design of the demonstrative platform. The platform will treat zootechnical manures from swine and bovine origin, at least in the maximum loading condition (10 swine and 1 bovine heads). The total incoming flow rate has been determined by considering the average production of manures and sewages2 multiplied by the medium weight of each species.17 A similar approach has been regarded to calculate loads and concentrations of main macropollutants, by considering the total nitrogen net of the amount in straw and stripped ammonia (Table 4). By considering data referred to head per day, it is Table 4. Average Production of Zootechnical Mixtures and Nutrients Concentrations species
dejections, m3 t−1 yr−1
weight, t head−1
swine bovine
39 52
0.09 0.35
3
−1
Q, m yr 3.5 18.2
Q, L d
TN, mg L−1
TP, mg L−1
9.7 49.8
5113 2338
1322 1059
−1
possible to determine loads and discharge values with the variability of the incoming flow rate in terms of treated amount and characterization. To evaluate the best treatment techniques and design, the main physic-chemical parameters to be determined are the dry content (TS), the organic compound (TVS), the suspended solids (TSS), the partition between soluble (TNs) and particulate (TNp) nitrogen, the total (COD), soluble (CODs), and particulate (CODp) chemical
Figure 4. Pig and cattle slurry characterization. 5493
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Table 5. Mass BalancesAD and DW Units AD DW
swine matrix
Q, L d−1
COD, g d−1
TS, g d−1
TVS, g d−1
TFS, g d−1
TKN, g d−1
TNp, g d−1
TNs, g d−1
in out in out-dry matter out-supernatant bovine matrix
97 97 190 28 162 Q, L d−1
10476 6986 7848 7452 396 COD, g d−1
8730 6986 7920 7524 396 TS, g d−1
5820 4076 4683 4449 234 TVS, g d−1
2910 2910 3237 3075 162 TFS, g d−1
496 496 511 141 370 TKN, g d−1
124 87 102 97 5 TNp, g d−1
372 409 409 44 365 TNs, g d−1
63 63 63 24 39
8033 6837 6837 6495 342
8786 6837 6837 6495 342
6508 4558 4558 4330 228
2278 2278 2278 2164 114
131 131 131 86 45
106 75 75 71 4
24 56 56 15 41
AD
in out in out-dry matter out-supernatant
DW
Table 6. Characteristics of the Fermentation Unit volume, m3
Qin, L d−1
TS, kg d−1
water, kg d−1
TSsolution, %
waterdil, kg d−1
Qtot, L d−1
HRT, d
0.2
8.0
2.8
5.2
12
15.4
23.4
8.6
have TS contents of about 8 kg d−1, with similar amounts of suspended and fixed solids. Anaerobic Digestion and Dewatering Processes. To ensure a HRT of 18 d, part of the dewatered excess sludge from the cattle dejection (calculated on the removal percentage of 95% and the effluent dry content equal to 6%) is digested with the influent flow rate. The AD process was verified by guaranteeing a TVS removal equal to 30%, calculated on the basis of the specific gas production settled on 0.28 N m3 kg TVS−1. The ratio between TNp and total volatile solids was maintained constant and equal to 0.02 for both the species: the conversion of TNp to ammonia, gained from the reduction of TVS, permitted to define a percentage of the effluent TNs on TKN equal to 83% and 43% for swine and cattle slurries, respectively, compared to the incoming values of 75% and 19%. The next dehydration process in water line is applied to both the digested slurries and the mass balances are calculated by settling a removal percentage of 95%, with a final dry content of 27%. Concerning the sludge line, besides the bovine excess biomass production, the dewatering process will be applied also to the swine slurry flow rate, which supernatant will be sent to the AC biological compartment. The TS and TKN loads were determined on the basis of the incoming values multiplied for the removal percentages. It is important to outline that the low value of COD/TN ratio (1.07 for swine slurry) suggests the necessity to provide the biological process with a dosage of external carbon to enhance the denitrification performances. Table 5 summarizes the influent (including the excess sludge part for the cattle slurry) and effluent characteristics of each dejection. Alternate Cycles Biological Process. The incoming dewatered flow rate will include part of the dehydrated excess sludge of swine and cattle matrixes. To obtain elevated TN removal performances, the AC process was sized by assuming the nitrification kinetics coefficient Kn at 10 °C of 0.13 kg NH4−N kg MLVSS−1 d−1, while the denitrification one was chosen equal to 0.07 kg NOx−N kg MLVSS−1 d−1. The excess sludge flow rate (0.09 m3 d−1) was determined by settling the sludge retention time to 15 d and the concentrations of the biological and recycle biomass equal to 5 kg m−3 and 10 kg m−3, respectively. Hence, it has been possible to calculate the ratio between the active biomass and the available substrate F/ M (equal to 0.04 and 0.05 kg COD kg MLVSS−1 for swine and
bovine slurry) and by assuming the N%TS value of 2.5, referring to MLVSS/MLSS, Xr, and Qw parameters, it was possible to obtain the TN load in the sludge (0.015 kg d−1). The nitrogen load to remove TNR (0.23 kg d−1 for pig and 0.03 kg d−1 for cattle slurry) was calculated as the difference between the incoming amount and the effluent one, obtained by assuming a removal efficiency of 50%. The time length of nitrification and denitrification phases for swine slurry, equal to 15 and 28 h, suggested the necessity to reduce the amount of heads to treat to 6. Regarding the bovine dejection, a duration of 4 and 3 h was sufficient to obtain a complete removal efficiency. Further, the time lengths for the aerobic and the anoxic phases were determined by dividing the nitrogen load to the biomass concentration, the MLVSS/MLSS, and the Kn values. Concerning the swine supernatants to the AC unit, the low COD/TN ratio of 1.07 suggest the necessity to support the denitrification phase with external carbon dosage. Configuration of the Fermentation Unit. The rapidly biodegradable carbon could be increased by providing the process with a stage of crop fermentation, in which TVS/TS and TS% values are equal to 0.95 and 35%, respectively. To raise the COD/TN ratio (influent to the AC reactor) up to 3.5, an effluent COD load from the fermentation unit of 0.6 kg COD d−1 is required to remove a nitrogen load of 0.23 kg TNR d−1. The substrate to furnish has been estimated equal to 2.7 kg TVS d−1 by considering the hydrolysis constant of 0.3 kg COD kg MLVSS−1 d−1. It is important to highlight that the fermentation process determines an increment of ammonia (0.05 kg NH4−N d−1) that requires a COD amount, to convert the ammonia in gas nitrogen, of about 0.11 kg COD d−1. Hence, the total external carbon to supply is equal to 0.7 kg COD d−1, which corresponds to 8 kg d−1 of crops (TS% of 35). Considering these values, the required area to cultivate crops should approximately be equal to 0.07 ha yr−1. To respect the spreading limit of 170 kg TN ha−1 yr−1 for 10 pig heads, for which an area of 0.06 ha yr−1 would be required, the land to be designated for crop culture would be approximately the 13% of the total. In Table 6, characteristics of the fermentation unit are summarized. The effluent from the fermentation unit could be dehydrated (E% of 95%, TS of 45%), to redigest the solid amount while the supernatants, containing the soluble carbon fraction, will support the biological process. The obtained supernatants, about 19 L d−1, would have a soluble COD 5494
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Figure 5. Pig and cattle treatment linesTN loads (g d−1 head−1) and TN removal efficiencies.
content of 0.48 kg d−1; hence, a grater quantity of crops need to be fermented. In the experimental phase, the denitrification stage of the biological process will be provided with the supply of RBCOD from sodium acetate. The estimated application of the fermentation unit has to be investigated to verify the effective denitrification capacity of the biomass, supported by hydrolyzed substrate, and the economic advantage for the land owner both. Design Phase of the Demonstrative Platform: Removal Efficiencies. Figure 5 summarizes the maximum nitrogen loads for each unit in swine and cattle dejections treatment lines, respectively. In particular, the TN removal efficiency is reported, together with the influent and effluent nitrogen loads per treated head (g TN d−1 head−1). Concerning the pig slurry treatment line, in view of the incoming TN load of 49.6 g d−1 head−1, a reduction of 50% was estimated, with effluent nitrogen from the clarifier equal to 12.3 g d−1 head−1 and from the drying unit of 14.1 g d−1 head−1. With reference to cattle dejections, the AC process guaranteed the complete nitrogen removal while the amount of TN in the effluent from the DW was estimated equal to 86.4 g d−1 head−1.
the more operative complex Anammox system. From the operative results coming from the pilot scale study, a demonstrative platform was designed to evaluate the possibility of a full scale application. The mass balances for each phase (AD, DW, and AC) were calculated on the basis of the influent and effluent concentrations parameters to define the reduction percentages. Referring to bovine slurry, the nitrogen balance of each operation unit determined a reduction from 116.4 g TN d−1 head−1 to 86.4 g d−1head−1 in the DW phase, to highlight the greater amount of particulate nitrogen rather than the soluble one. Complete nitrogen removal performed by the AC process with time lengths of nitrification and denitrification phases respectively of 4 and 3 h was estimated. Concerning the pig slurry treatment line, effluent nitrogen loads equal to 12.3 g d−1 head−1, from the AC reactor, and 14.1 g d−1 head−1 in the solid dehydrated sludge were gained. Moreover, for the swine treatment line, an increase of the COD/TN ratio is required: the RBCOD could be supplied with the dosage of 0.7 kg COD d−1 (equal to 8 kg d−1 of crops) from the dewatered supernatant of fermented silomais. Obviously the reliability of the adoption of a fermentation unit is related to the technical and economical situation of farms.
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CONCLUSION The present study evaluated the removal efficiencies of the biological AC post treatment for anaerobic digested supernatants, in a pilot scale reactor continuously fed. The high performances enabled to obtain average effluent concentrations of 61.6 mg NH4−N L1− and 92.4 mg TN L−1, with removal percentages of 95% for both the nitrogen forms. The AUR and NUR batch tests on the biomass demonstrate the main removal mechanism related with the nitrites transformation. The performances obtained, similar to ones of other literature processes, could be consider a relevant prospective to treat flow characterized by high nutrient loads in a continuous biological way compared with the traditional discontinuous SBR and with
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the Fondazione Cariverona for the support to the scientific activity, within the project “Biomasse di oggi e di domani: dai reflui zootecnici e dalle microalghe un contributo all’agricoltura sostenibile e all’energia rinnovabile”. 5495
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NOMENCLATURE AC = alternate cycle biological advanced process AD = anaerobic digestion AUR = ammonia uptake rate test CivSup = civil supernatant from urban wastewater treatment plant (%) COD = chemical oxygen demand concentration (mg L−1) CODp = particle chemical oxygen demand concentration (mg L−1) CODs = soluble chemical oxygen demand concentration (mg L−1) DO = dissolved oxygen concentration (mg L−1) DW = dewatering unit F/M = COD influent load on volatile biomass amount ratio (kg COD kg MLVSS−1) HRT = hydraulic retention time (d) kd = denitrification rate (kg NOx−Nremoved kg MLVSS−1 d−1) kn = nitrification rate (kg NOx−Nproduced kg MLVSS−1 d−1) MLSS = mixed liquor suspended solids concentration (mg L−1) MLVSS = mixed liquor volatile suspended solids concentration (mg L−1) N%TS = nitrogen percentage on total solid (%) NH4−N = ammonia concentration concentration (mg L−1) NLR = Nitrogen Loading Rate (kg TN m−3 d−1) NOx−N = nitrite and nitrate concentrations (mg L−1) NO2−N = nitrite concentrations (mg L−1) NO3−N = nitrate concentrations (mg L−1) NUR = nitrogen uptake rate test OLR = organic loading rate (kg COD m−3 d−1) ORP = oxidation reduction potential (mV) PO4−P = orthophosphates concentration (mg L−1) Qin = influent flow (L d−1) Qw = waste flow (L d−1) RBCOD = rapidly biodegradable carbon (mg L−1) TFS = total fixed solids concentration (mg L−1) TKN = total Kjeldal nitrogen concentration (mg L−1) TN = total nitrogen concentration (mg L−1) TNp = particle total nitrogen concentration (mg L−1) TNR = total nitrogen load removed (kg d−1) TNs = soluble total nitrogen concentration (mg L−1) TP = total phosporous concentration (mg L−1) TS = total solids concentration (mg L−1) TS% = total solids percentage (%) TSS = total suspended solids concentration (mg L−1) TVS = total volatile solids concentration (mg L−1) Xr = MLSS concentration in recyrcle flow (mg L−1) ZooSup = zootechnical supernatant from anaerobic digestion (%)
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REFERENCES
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dx.doi.org/10.1021/ie202691n | Ind. Eng. Chem. Res. 2012, 51, 5490−5496
