Anaerobic co-digestion of sludge with other organic wastes and ...

D. Bolzonella*, P. Pavan**, P. Battistoni*** and F. Cecchi* *Department of Science and Technology, University of Verona, Strada Le Grazie, 15. I-37134 Verona, Italy (E-mail: [email protected]) **Department of Environmental Sciences, University of Venice, Dorsoduro 2137. I-30123 Venice, Italy ***Institute of Hydraulics, Polytechnic University of Marche, via Brecce Bianche. I-60100 Ancona, Italy Abstract This paper deals with the performances obtained in full scale anaerobic digesters co-digesting waste activated sludge from biological nutrients removal wastewater treatment plants, together with different types of organic wastes (solid and liquid). Results showed that the biogas production can be increased from 4,000 to some 18,000 m3 per month when treating some 3–5 tons per day of organic municipal solid waste together with waste activated sludge. On the other hand, the specific biogas production was improved, passing from 0.3 to 0.5 m3 per kgVS fed the reactor, when treating liquid effluents from cheese factories. The addition of the co-substrates gave minimal increases in the organic loading rate while the hydraulic retention time remained constant. Further, the potentiality of the struvite crystallisation process for treating anaerobic supernatant rich in nitrogen and phosphorus was studied: 80% removal of phosphorus was observed in all the tested conditions. In conclusion, a possible layout is proposed for designing or upgrading wastewater treatment plants for biological nutrients removal process. Keywords Anaerobic co-digestion; organic wastes; struvite crystallisation; waste activated sludge

Introduction

Water Science & Technology Vol 53 No 12 pp 177–186 Q IWA Publishing 2006

Anaerobic co-digestion of sludge with other organic wastes and phosphorus reclamation in wastewater treatment plants for biological nutrients removal

Biological nutrients removal (BNR) processes, either for nitrogen or both nitrogen and phosphorus removal, rely on the availability of easily biodegradable carbon in the wastewater to be treated. According to this evidence, primary sedimentation is generally absent in wastewater treatment plants (WWTPs) adopting the BNR processes to maintain the particulate fraction of the influent COD. Moreover, in order to preserve the nitrification capability of the activated sludge, high solids retention times (SRT) are applied in the activated sludge process (. 10 d) especially at low temperatures. As a consequence, a partial sludge stabilisation occurs in the activated sludge process and the anaerobic stabilisation of waste activated sludge (WAS) can result in low efficiency from both a processing and an economic standpoint: observed SGP are generally , 0:2 m3 kg VS21 fed and removal of volatile solids is 20–30% (Bolzonella et al., 2002, 2005). In order to improve the performances of anaerobic digesters, the co-digestion of waste activated sludge together with other organic wastes is a common practice adopted in wastewater treatment plants (Cecchi et al., 1994; Rintala and Jarvinen, 1996; Oleszkiewicz and PoggiVaraldo, 1997; Pavan et al., 2000; Battistoni et al., 2002b; Krupp et al., 2005). Previous studies showed that co-digestion of sludge together with organic municipal solid wastes allows for the increase of biogas production from some 0.2 m3 kgVS21 up to 0.6 m3 kgVS21 fed to the digester, depending on the quality and quantity of co-digested wastes (Sosnowski et al., 2003). This can be a valid solution to improve the performances of some 36,000 anaerobic digesters now operating within the European Union (Mata-Alvarez et al., 2000) so to improve the energetic balances of the plants or to exploit the green certificates for the production of doi: 10.2166/wst.2006.420

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energy from renewable sources. However, the presence of high concentration of phosphorus in sludge from BNR processes (generally . 3%, up to 6% of total solids) or in co-digested organic wastes determines some problems related to the presence of nitrogen and phosphorus in the rejected water during dewatering of the anaerobic digester sludge (Mavinic et al., 1998; Battistoni et al., 2002a). This stream should be conveniently treated to block the recycle of nutrients to the wastewater treatment line. The struvite crystallisation process allows for this aim (Battistoni et al., 1997). This paper deals with the performances obtained applying the co-digestion process in two Italian WWTPs where the biological nutrients removal processes are performed. Moreover, the potentiality of the struvite crystallisation process for treating anaerobic supernatants rich in nitrogen and phosphorus is reported. In conclusion, a possible layout for designing or up-grading WWTPs applying BNR processes is proposed. Material and methods

The application of the anaerobic co-digestion process was studied in two large Italian WWTPs where the BNR processes are applied: Treviso WWTP (70,000 PE), north-east Italy, and Jesi WWTP (100,000 PE), central Italy. The main characteristics of the plants are reported below. Treviso wastewater treatment plant

Treviso WWTP (70,000 PE) co-treats wastewaters and organic municipal solid wastes (Cecchi et al., 1994; Pavan et al., 2000). It treats 14,000 m3 d21 of wastewater without primary sedimentation in a Johannesburg configuration process for C, N and P biological removal and up to 20 tons per day of municipal organic wastes from separate collection (markets, restaurants, canteens). The volumes of the different sectors of the activated sludge process can be varied to reach different efficiencies in N and/or P removal. Waste activated sludge is thickened and treated in a 2,200 m3 mesophilic anaerobic digester together with pretreated organic solid wastes. The organic wastes are pretreated in a dedicated line for metals and plastics removal and shredding of the organic material (some 20 –25% total solids). This is then sent to a mixer/separator where it is diluted to a solid content of 7–8%: floating materials and bottom residues are withdrawn. The diluted material can be then: (a) mixed with the waste activated sludge and fed to the anaerobic digester; (b) prefermented in a complete stirred tank reactor (HRT ¼ 3–6 d, OLR 20 –60 kg TVS m23 d21), mixed with waste activated sludge and fed to the anaerobic digester; (c) prefermented and pressed to obtain a liquid fraction to be used as carbon source in the BNR processes and a solid fraction to be sent to the anaerobic digester together with the waste activated sludge. Depending on the chosen pathway, either nutrients removal or energy recovery is enhanced (Pavan et al., 2000, 2004). Table 1 shows the main characteristics of Treviso WWTP. Digested sludge produced is dewatered in a belt-press and used for agricultural purposes. Jesi wastewater treatment plant

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In Jesi WWTP (60,000 PE), primary settling is avoided and carbon is preserved to perform a good biological nutrients removal. With specific reference to the sludge treatment line, wasted activated sludge is gravitationally thickened and sent to anaerobic digestion. Then, digested sludge is post-thickened in a gravitational tank and dewatered by belt-press. The obtained sludge is then disposed of in landfills. Because of the limited biogas production and the consequent problems of energy balance, liquid organic wastes such as dairy by-products and residuals from olive oil making factories are co-digested with WAS.

Table 1 Volumes and operational conditions for Treviso and Jesi wastewater treatment plants Parameter

Sludge treatment

Flowrate, m3 d21 COD, kg d21 TN, kg d21 TP, kg d21 Section Anaerobic section, m3 Activated sludge recirculation flowrate, m3 d21 Anoxic section, m3 Recycle activated sludge, m3 d21 Oxidation/nitrification section, m3 Biological reactor total volume, m3 Hydraulic retention time, h Solid retention time, d Sludge loading rate, kg BOD5kg MLSS21 d21 Secondary settler area, m2 Surface loading rate, m3 m22 h21 Disinfection/contact tank, m3 Pre-thickener, m3 Anaerobic digester, m3

14,000 2,200 250 25 400 –1,200 Up to 23,760 1,600– 1,200 Up to 37,440 5,500 9,000 15 15 0.125 1.300 0.45 250 210 2,200

Jesi WWTP

13,000 7,200 720 72 210 26,000 4,248 39,000 6,172 10,420 19 15 0.14 1.432 0.38 895 352 1,500


Line Wastewater treatment

Treviso WWTP

Struvite crystallization process

The struvite crystallisation process (SCP) without addition of chemicals to block phosphorus is a promising technology applied for the treatment of anaerobic supernatants (Battistoni et al., 2005b). A demonstrative plant has been operating since 1999 at Treviso WWTP (Battistoni et al., 2005a). The plant (Figure 1 and Table 2) is fed with anaerobic supernatant produced by belt-press dewatering of anaerobically digested sludge. The feeding flowrate is firstly stripped with air in a stripping column to increase pH, then it flows to the de-aeration column, and is then pumped into a 1 m3 volume fluidised bed reactor (FBR). The SCP unit treats up to 2.0 m3 h21 of anaerobic supernatant in a continuous mode. A Dortmund apparatus at the top of the FBR avoids the wash-out of fine materials (linear velocity of 6 m h21). The effluent of the FBR is then recycled to the stripping column and the final effluent is obtained from the de-aeration column discharge.

Figure 1 Scheme of the demonstrative plant for struvite crystallisation process (SCP)

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Table 2 Main characteristics and geometric dimension of the plant for struvite crystallisation process Section

Volume, geometry, flowrate


F ¼ 0.9 m, Vtot ¼ 1.3 m3 F ¼ 1.6 m, Vtot ¼ 4.7 m3 Vtot ¼ 48 m3 0.8 –4.9 m3/h Vtot ¼ 1.33 m3 Vtot ¼ 0.53 m3 Vtot ¼ 0.85 m3 Vtot ¼ 0.80 m3

Mixer Decanter Equalisation basin Pumps P2 Stripper De-aeration column Fluidised bed reactor Dortmund

Results and discussion Anaerobic co-digestion at Treviso wastewater treatment plant

The plant started its activity in winter 2000. At the beginning, the digester was used only for the digestion of waste activated sludge and was started-up in the mesophilic range of temperature (37 8C), using the activated sludge produced in the plant as inoculum (i.e., seed).

Digestion of waste activated sludge (secondary sludge). The operational conditions and yields obtained in steady state conditions when treating waste activated sludge are summarised in Table 3, column “WAS alone”. The HRT applied was in the range normally used for wasted sludge (around 30 –40 d) while the organic loading rate can be considered low (, 1 kg TVS m23 d21). Total solids concentration of fed sludge was some 3%: this was prethickened by a conventional gravitational thickener. The application of Table 3 Digester operational conditions and yields observed during the anaerobic digestion of waste activated sludge and in co-digestion with the pretreated or the solid phase of fermented OFMSW (average values obtained in at least three HRTs) Item

180

Parameter

Operational Temperature conditions HRT Total OLR Sludge OLR OFMSW OLR Feed TS TVS TCOD VFA pH TKN Digester TS TVS TCOD VFA pH TA P-PO4 NH3-N Biogas GPR GPROFMSW SGP SGPOFMSW GP CH4

Unit

8C

WAS only

34.5

d 37.2 kg TVS m23 d21 0.53 23 21 kg TVS m d 0.53 kg TVS m23 d21 – g kg21 36.0 %TS 62 mg COD kg TS21 789 mg COD L21 – – 6.7 g N kg TS21 40 g kg 21 25.8 %TS 54 mg COD kg TS21 696 21 mg COD L , 100 – 6.90 mg CaCO3 L21 1,865 mg P L21 90 mg N L21 350 m3 m23 d21 0.10 m3 m23 d21 – m3 kg TVS21 0.13 fed m3 kg TVS21 – fed m3 month21 4,791 % 68.0

WAS 1 OFMSW

36.3 35.6 0.78 0.46 0.32 41 67 1,515 1,743 6.1 30.9 30 56 1,199 109 7.2 3,058 113 406 0.34 0.25 0.43 0.77 17,460 64.0

WAS 1 solid fermented OFMSW

35.0 40.0 0.68 0.33 0.35 40 58 – – 6.7 45 34 52 – 290 7.0 2,117 – 371 0.21 0.11 0.27 0.67 12,600 67.0


these operational conditions allowed for reaching a steady state condition in some 60 d. The specific yield in terms of biogas production was some 0.13 m3kg TVS21, which is low but appropriate considering the kind of sludge treated and the solid retention time applied in the activated sludge process (. 20 d; Bolzonella et al., 2005). Biogas yield was some 4,800 m3 per month (68% CH4), insufficient to cover the energy requirements for the heating of the anaerobic digester, especially during winter months. The evolution of the stability parameters was quick and typical values were reached in 1 HRT. In particular, pH was stable at 6.9 during the whole period. Alkalinity increased up to 1,900 mg CaCO3 L21 during the first 60 d, while VFA (as the sum of C1 –C5 compounds) content was always below 100–200 mg L21. Therefore, it can be concluded that the system was completely stable and successfully started-up but yields were insufficient. Table 3, column “WAS only”, reports the operational conditions and yields of the process. After a few months, the co-digestion process was applied to enhance the digester yields. The digester performances have been studied in two different operational conditions: co-digestion of WAS with pre-treated OFMSW and co-digestion of WAS with the solid fraction of the prefermented OFMSW. Co-digestion of secondary sludge and OFMSW. Received OFMSW, up to 10 tons per day, were pretreated in a patented sorting line. The characteristics of the OFMSW after pretreatment are summarised in Table 4. As can be seen, the selection line allowed for obtaining an excellent effluent in terms of biodegradability: the material was highly biodegradable: TVS/TS ¼ 87%, COD/TS ¼ 2.1 and 20 g L21 of soluble COD (Pavan et al., 2004). The operational conditions and yields of the process in this situation are those summarised in Table 3, fifth column. The addition of the OFMSW to secondary sludge led to a notable increase of digester yields. The GPR passed from 0.10 m3 m23 d21, obtained with sludge alone, to 0.34 m3 m23 d21. In specific terms, the SGP increased up to 0.43 m3 kg TVS21 fed, while, on a monthly basis, the produced biogas increased from some 4,800 m3 per month up to 17,500 m3 per month with a clear benefit from an energetic standpoint. With reference to the stability parameters, pH remained in the normal operational ranges, as well as VFAs that showed concentration of 100 mg L21 or less. Total alkalinity was clearly higher than that observed when digesting WAS alone: it passed from 1,800 to 3,000 mg CaCO3 L21, increasing the buffer capacity of the system.

Co-digestion of secondary sludge and solid fraction of prefermented OFMSW. In this configuration, OFMSW was prefermented so to obtain a liquid phase useful for enhancing the BNR processes in the activated sludge process (Pavan et al., 2000; Bolzonella et al., 2001). The solid residue (20% total solids) of the fermented OFMSW, Table 4 OFMSW characteristics after pretreatment Item

Average

Std. Dev.

Max

Min

TS, g kg21 TVS, %TS TCOD, g kg TS21 SCOD, g kg TS21 VFA, mg COD L21 TKN, g N kg TS21 NH3-N, g N L21 TP, g P L21

90 87 2,144 222 1,123 33 49.3 13

17 4 422 78 314 2 8.6 4

105 93 2,744 278 1,485 38 50.4 19

51 79 1,800 111 740 30 48.0 8

181


obtained after screwing, was mixed together with secondary sludge and sent to the anaerobic digester (final solids at 5%). The results of the experimentation are shown in Table 3, last column. Passing from one condition to the other, the biogas production decreased in terms of SGP (from 0.43 to 0:27 m3 kg TVS21 fed ) since an important part of the biodegradable COD was transferred to the liquid stream used in the anaerobic and/or anoxic steps for nutrients removal. Biogas production decreased from 17,460 to 12,600 m3 per month while all the other operational parameters remained constant. Concerning the economical feasibility of the proposed process, a complete evaluation was performed considering different capacities for the OFMSW treatment. The investment cost for a treatment line with a capacity of 15 tons per day was some 1.5 million Euro, while treatment costs at industrial scale were some 40 –50 e/ton when treating more than 50 tons per week of organic wastes. This was because of the low energy input requested by the treatment line: , 50 kWh per treated ton of waste. Further, because of these results, a co-generation unit for power and heat is going to be installed at Treviso WWTP to obtain green certificates for the production of electric energy from renewable sources.

Anaerobic co-digestion at Jesi wastewater treatment plant

Depending on the season, two different liquid organic wastes, olive oil mill effluent (OME) and cheese dairy effluent (CDE), are co-digested together with waste activated sludge in the mesophilic anaerobic digester at Jesi wastewater treatment plant. Typical characteristics of the two substrates are reported in Table 5. Both the substrates have been tested in these years to enhance the digester performances. In this paper, the results related to the co-digestion of waste activated sludge with CDE are reported. During the experimental trials, the flow of prethickened waste activated sludge was some 45 m3 per day, while the dairy wastes ranged between 0 and 10 ton per day during a period of some 9 months. Wasted sludge was prethickened to a total solid concentration of 4–6%. Therefore, the HRT of the reactor always remained at very high values (more than 36 d): the addition of CDE determined the reduction of the HRT to a minimum of 30 d. The reactor temperature, which was difficult to maintain in the mesophilic range when only WAS was fed to the reactor, rose to 37 8C and was easily kept constant. In fact, the variation in biogas production was clearly related to the feeding of dairy by-products (see Figure 2): peaks in biogas production are clearly related to CDE feeding; in particular, the addition of some 7– 8 tons of CDE improved biogas production from 300 to 600 m3 per day. The co-digestion process showed good performances in terms of volatile solids removal (see Table 6) and the specific biogas production was excellent (average SGP ¼ 0.7 Nm3 per kg of volatile solid fed). The average characteristics of the anaerobic supernatants observed during the addition of CDE are shown in Table 7. The soluble COD was always below 1,200 mg L21, a value similar to that observed when only WAS was digested, demonstrating that food Table 5 Average chemical physical characteristics of OME and CDE wastes

182

Liquid waste

pH

COD

TS

TVS/TS

TN

TP

SGP

OME CDE

5 5.8

g L21 125 75

g L21 102 73

0.80 0.84

mg L21 803 1,465

mg L21 357 335

Nm3 kg COD21 0.35† 0.42 –0.56*

OME: olive oil mill effluent, CDE cheese diary effluent; *Yilmazer and Yenigun (1999); †Beccari et al. (2001)

700

CDE load

600

12 10

400

8

300

6

200

4

100

2

– 20/12

30/12

9/1

19/1

29/1

8/2

18/2

– 28/2

Days


500

CDE ton

Biogas m3/d

14

Q biogas

Figure 2 Biogas production and organic loading dairy effluent

wastes, such as CDE, are easily degradable and no accumulation of acids occurred when this stream is properly fed to the reactor. In fact, phenomena of acidification were not observed in the reactor during the co-digestion periods: pH remained stable, in the range of 7–7.3. The lowest value of 6.94 was registered with an OLR of 0.93 kg COD m23 d21. The ammonia concentration was some 400 mg L21, a typical value for anaerobic supernatants deriving from the digestion of waste activated sludge, while phosphates were in the range of 40 –60 mg P L21. Obviously, these streams of nutrients are an extra load to be treated in the wastewater treatment line; therefore, before applying the co-digestion process, an analysis of the activated sludge potentiality should be carried out carefully. Application of the struvite crystallisation process

It was clear from the examples reported above that the anaerobic co-digestion of waste activated sludge from BNR processes together with other organic substrates determines an anaerobic supernatant rich in nutrients (some 400 mg N L21 of ammonia and up to 100 mg P L21 of phosphates) which are recycled to the wastewater treatment line. These loads of nutrients can be conveniently blocked by applying a struvite crystallisation process (SCP) where nitrogen and phosphorus are fixed in struvite crystals together with magnesium (MgNH4PO4). The SCP has been widely applied for the treatment of anaerobic supernatants coming from digestion of WAS and co-digestion of WAS together with solid organic wastes; however, its application can be performed for any anaerobic supernatant whose chemicophysical characteristics are similar to these: phosphorus concentration higher than 30–50 mg P L21, ammonia . 300 mg N L21, magnesium . stoichiometric requirement and alkalinity . 900 mg CaCO3 L21. The average performances observed in a demonstrative plant experimentation are distinguished between nucleation efficiency (h) and Table 6 Operational conditions and efficiencies in co-digestion of waste activated sludge with cheese dairy effluent co-digestion at Jesi wastewater treatment plant

Jan-Feb-Mar Apr-May-Jun Jul-Aug

Influent VS

Influent

Effluent TVS

TVS/Tsout

kg VS d21

VS CDE kg VS d21

kg VS d21

%

684 752 985

234 301 291

526 608 837

55.9 54.0 41.2

VS removal %

HRT d

Treactor 8C

43.3 42.3 34.6

49 45 36

30.9 35.5 35.8

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Table 7 Characteristics of the digester supernatants during the co-digestion of WAS and CDE Parameter

Unit

pH SCOD PO4-P NH4-N

mg L21 mg L21 mg L21

Value

7.0– 7.3 1,200 40– 60 400


Table 8 Operational conditions for the SCP process Feeding flowrate m3 h21

1 –4

Qr m3 h21

Qair m3 h21

Bed porosity

V0 10 ml L21

18

0–20

0.6 –0.98

50 –100

pH

X%

h%

L%

7.9– 8.2 75–80 70 –78 2–5

conversion (X) (see Table 8). Loss of fine materials (suspended solids) is always lower than 2–5%, not requiring a filtration unit for the effluent treatment (Battistoni et al., 2002a). The operational costs determined by the application of the SCP are essentially due to energy consumptions for pumping devices (Battistoni et al., 2005b). Following the two possibilities to perform the phosphorus crystallisation, with and without seed material, the operational costs can be distinguished. When using quartz sand as seed material, costs are some 0.24 e per m3; when the auto-nucleation is obtained, a lower cost can be reached (down to 0.16 e per m3). Both costs analysis do not consider the possibility of selling struvite as a fertiliser. The comparison of these costs with those published for the Crystallactorw process (0.57 e per m3, Van Dijk and Braakensiek, 1984; mainly due to addition of chemicals) reveals the FBR process as the cheaper way to remove and reclaim P or P and N from anaerobic supernatants.

Conclusions

184

Co-digestion of waste activated sludge with other organic substrates is a feasible way to improve the performances of anaerobic digesters in WWTPs where BNR processes are applied, provided that the recycled nutrients can be treated in the wastewater treatment section. Results of experimentations where waste activated sludge was co-digested with other solid or liquid organic wastes showed that: † the addition of the organic fraction of municipal solid wastes with a sludge/OFMSW ratio of 60:40 on a TVS basis allowed for an increase of the organic loading rate up to 1 kg VS per m3 per day and thus in biogas production from some 0.13 m3 kg VS21, when only waste activated sludge was digested, up to 0.43 m3 kg VS21 in the case of co-digestion; † the investment cost for a treatment line with a capacity of 15 tons per day of OFMSW was some 1.5 million Euro while treatment costs were some 40 –50 e/ton when treating some 50 tons per week of organic wastes; this was because of the low energy input required by the treatment line: , 50 kWh per treated ton of waste; † the anaerobic co-digestion of biological sludge with cheese dairy effluent (CDE) showed the possibility of producing some 0.35–0.45 m3 of biogas per kg of COD fed, being the hydraulic retention time maintained constant; † phosphorus and, partially, nitrogen can be reclaimed in form of struvite or other phosphorus minerals (i.e. hydroxyl-apatite) from anaerobic supernatant by applying the


struvite crystallisation process. This allowed for 80% removal of phosphorus from anaerobic supernatants at industrial costs of some 0.16 –0.24 e per m3 treated. According to the data reported in this paper, the proposed layout for the design or upgrading of an effective BNR process is the following: † elimination of primary sedimentation to preserve the COD and improve biological nutrients removal; † addition of a liquid or solid organic waste as co-substrate for anaerobic digestion of waste activated sludge to improve the energetic balance of the anaerobic digester and produced energy from renewable sources; † block of excess phosphorus and nitrogen in the digestate supernatant by means of the struvite crystallisation process.

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