Paolo Pavan

Phosphate removal in anaerobic liquors by struvite crystallization without addition of chemicals. Preliminary results.

Abstract The feasibility of phosphate removal from the supernatant of anaerobically digested sludge by struvite (MAP, MgNH 4 PO 4 ) crystallization in a fluidized-bed reactor (FBR) was studied. Quartz sand was used as seed material. Three successive steps were investigated to highlight the process: 1, natural aging of the supernatant, with phosphate concentrations in the range 18–164 mg/l, which gave good crystallization of struvite within 3 days; 2, the inhibiting effect of Mg and bicarbonate ions on hydroxyapatite (HAP, Ca 5 OH(PO 4 ) 3 ) formation, which leads to struvite formation under supersaturation conditions; and 3, phosphate removal via the process of complete crystallization on the seed material, and which is obtained by means of external continuous aeration. All runs were performed without the addition of chemicals.

Anaerobic digestion of the Barcelona central food market organic wastes: Experimental study

Abstract Experiments carried out to obtain data for a preliminary design of an anaerobic digestion plant treating the organic wastes coming from a large food market are described. Four hydraulic retention times (8, 12, 14 and 20 days) were tested in 3-litre laboratory digesters. The methane yields were high (around 0·478 m3 CH4/kg VS added). Kinetic analysis, using a first-order model, resulted in a kinetic constant of 3·1 day−1 and an ultimate methane yield of 0·489 CH4/kg VS added. The biogeradability of the soluble and non-soluble fractions of the Volatile Solids (VS) is discussed.

Optimization of two-phase thermophilic anaerobic digestion of biowaste for hydrogen and methane production through reject water recirculation.

The optimization of a two-phase thermophilic anaerobic process treating biowaste for hydrogen and methane production was carried out at pilot scale using two stirred reactors (CSTRs) and without any physical/chemical pre-treatment of inoculum. During the experiment the hydrogen production at low hydraulic retention time (3d) was tested, both with and without reject water recirculation and at two organic loading rate (16 and 21 kgTVS/m3 d). The better yields were obtained with recirculation where the pH reached an optimal value (5.5) thanks to the buffering capacity of the recycle stream. The specific gas production of the first reactor was 51 l/kgVS(fed) and H2 content in biogas 37%. The mixture of gas obtained from the two reactors met the standards for the biohythane mix only when lower loading rate were applied to the first reactor, with a composition of 6.7% H2, 40.1% CO2 and 52.3% CH4 the overall SGP being 0.78 m3/kgVS(fed).

Phosphorus removal from a real anaerobic supernatant by struvite crystallization

In this paper the phosphorus removal from a real anaerobic supernatant through the crystallization of struvite and or hydroxyapatite was investigated. A comparison between experimental results on phosphorus crystallization carried out in a fluidized-bed reactor (FBR) on a bench-scale and on a half-scale plant is presented, together with a double saturational model able to describe all experimental results, independent of the different geometry of the reactors, the distinct contact times and the unlike products obtained. Experimental results show that removal efficiencies are very satisfactory, and the maximum phosphorus removal is of 80%.

Application of the IWA ADM1 model to simulate anaerobic co-digestion of organic waste with waste activated sludge in mesophilic condition.

Anaerobic digestion model no. 1 model of international water association was applied to a full scale anaerobic co-digestion process for the treatment of the organic fraction of municipal solid wastes along with activated sludge wastes originating from a municipal wastewater treatment plant. This operation was carried out in a digester of 2000 m(3) in volume. It is operates at an average hydraulic retention time of 26.9 days with an average organic loading rate of 1.01 kg TVS/m(3) day, at a temperature of 37 degrees C with an average gas production rate of 0.296 m(3)/m(3) day. The aim of the present study is to compare the results obtained from the simulation with the experimental values. The simulated results showed a good fit for pH, methane and carbon dioxide percentages, biogas volume, chemical oxygen demand, total volatile fatty acids, inorganic nitrogen and inorganic carbon.

Anaerobic co-digestion of sewage sludge: application to the macroalgae from the Venice lagoon

Abstract Possibilities of co-digestion of sewage sludge (SS) with other organic wastes are examined in this paper. Anaerobic co-digestion of macroalgae of the Venice lagoon (A) with SS, in wastewater treatment plants is studied in detail. This approach can contribute to the solution of the final disposal of the 50 000 m 3 of macrophytes harvested each season. These are mainly Ulva rigida and Gracilaria confervoides . In the experiments A and SS were mixed at different ratios (20–40% algae, TS basis) and fed to mesophilic (37°C) and thermophilic (55°C) digesters which operated at 11- to 15-day hydraulic retention times and 1.7–4.4 kgTVS/m 3 /day organic loading rates. It was concluded that the mesophilic co-digestion process is applicable with potentialities of around 30% of the present SS flow-rate. Thermophilic digestion is not possible, because of the inhibition of methanogens probably due to the activity of sulphate-reducers.

High rate mesophilic, thermophilic, and temperature phased anaerobic digestion of waste activated sludge: A pilot scale study

The paper reports the findings of a two-year pilot scale experimental trial for the mesophilic (35°C), thermophilic (55°C) and temperature phased (65+55°C) anaerobic digestion of waste activated sludge. During the mesophilic and thermophilic runs, the reactor operated at an organic loading rate of 2.2 kgVS/m(3)d and a hydraulic retention time of 20 days. In the temperature phased run, the first reactor operated at an organic loading rate of 15 kgVS/m(3)d and a hydraulic retention time of 2 days while the second reactor operated at an organic loading rate of 2.2 kgVS/m(3)d and a hydraulic retention time of 18 days (20 days for the whole temperature phased system). The performance of the reactor improved with increases in temperature. The COD removal increased from 35% in mesophilic conditions, to 45% in thermophilic conditions, and 55% in the two stage temperature phased system. As a consequence, the specific biogas production increased from 0.33 to 0.45 and to 0.49 m(3)/kgVS(fed) at 35, 55, and 65+55°C, respectively. The extreme thermophilic reactor working at 65°C showed a high hydrolytic capability and a specific yield of 0.33 g COD (soluble) per gVS(fed). The effluent of the extreme thermophilic reactor showed an average concentration of soluble COD and volatile fatty acids of 20 and 9 g/l, respectively. Acetic and propionic acids were the main compounds found in the acids mixture. Because of the improved digestion efficiency, organic nitrogen and phosphorus were solubilised in the bulk. Their concentration, however, did not increase as expected because of the formation of salts of hydroxyapatite and struvite inside the reactor.

Co-digestion of livestock effluents, energy crops and agro-waste: feeding and process optimization in mesophilic and thermophilic conditions.

In this study the optimization of the biogas yield from anaerobic co-digestion of manures and energy crops was carried out using four pilot scale CSTRs under different operating conditions. The effect on biogas yield of the partial substitution of energy crops with agro-waste was also investigated. For each substrate used during the continuous trials, BMP batch assays were also carried out to verify the maximum methane yield theoretically obtainable. Continuous operation results indicated that the co-digestion of manures, energy crops and agro-waste was viable at all operating conditions tested, with the greatest specific gas production of 0.54 m(3)/kg VS(fed) at an organic load rate of 2 kg TVS/m(3)(r)d consisting of 50% manure, 25% energy crops and 25% agro-waste on VS basis. No significant differences were observed between high and low loaded reactors suggesting the possibility of either improving the OLR in existing anaerobic reactors or reducing the design volumes of new reactors.

Effect of sludge age on the performance of a membrane bioreactor: influence on nutrient and metals removal

In this paper the effect of different SRT on the performances of an ultrafiltration pilot MBR operating with real wastewaters was tested to analyse both the ability of the system to remove nutrients and micropollutants and the possible decrease in waste sludge production. Increasing MLSS from 4 to 9 g/l reduced biomass production by 84% and increasing MLSS from 9 g/l to 17 g/l reduced biomass production by 75%. The progressive sludge mineralization was clear since the VSS decreases from 75% TSS to 52% TSS. The industrial fraction of the influent affected the denitrification in all the three periods. The effluent quality increased only when passing from short SRT to long SRT. Ag, Cd and Sn removal was >99% in all the runs. Cu removal was 72–89%. Hg removal was >90% while Pb had varying behavior due to its inconstant presence in the influent (50–65%). B and Se seemed not to be efficiently retained by the biomass in both experimental conditions (0–28% and 0–31%, respectively). Arsenic was a major concern (33–37%). The next step of the research will focus on the possibility of enhancing As removal by looking at alternative technologies to integrate with the UF process.

Volatile fatty acids production by mesophilic fermentation of mechanically-sorted urban organic wastes in a plug-flow reactor

Volatile fatty acids (VFA) production by anaerobic fermentation of municipal solid wastes was studied at pilot-plant level. A plug-flow reactor (80 l total volume), with and without concentrated sludge recirculation, was employed to digest an organic fraction of the municipal solid waste (OFMSW)-based substrate. This was composed of OFMSW mechanically-selected in a full-scale plant with OFMSW collected from fruit and vegetable markets and mixed in a way to obtain a feed mixture with a proportion of total volatile solids (TVS)/total solids (TS) of 60%. The fermentation process was studied at different retention times (RT) (from 8 h to 8 days) and at very high organic loading rates (OLR) (from 22·4 to 85·2 kg VS/m3 d) at mesophilic temperature (37 ± 2°C). Results showed an increase in VFA production at higher retention times (from 11·8 g/l at 2 days to 23·1 g/l at 6 days RT) when no recirculation was used. VFA production was also high when part of the digested sludge was recirculated (19·5 g/l at 2 days of hydraulic RT and 8 days of solid RT). Acetic and butyric were the acids produced in the largest quantities.