Iwa international Specialist Conference

K Stamatelatou¹, K. Dravillas and G. Lyberatos²

Department of Chemical Engineering, University of Patras, GR-26500 Patras, GREECE

e-mails: ^[e-mail], ² [e-mail] (author for correspondence)

Tel: [phone], fax: [phone]

Abstract

Sweet sorghum is an energy crop, often cultivated to recover energy in the form of ethanol, hydrogen etc by applying biological processes. These processes however produce a significant amount of sludge (bagasse) which contains mostly the recalcitrant unconverted portion of sorghum as well as microbial biomass. We propose a two-stage configuration for the anaerobic digestion of this type of industrial sludge. In this work, the sludge from the alcoholic fermentation of sweet sorghum following a distillation step (to remove the generated bioethanol) was subjected to anaerobic digestion for further biodegradation and energy production (methane). The solid portion of the sludge (9%) contributed mainly to the total organic load, although there was a significant organic load dissolved in the liquid portion too (28.7311.01 g/l). In order to optimize the process performance, it was decided to separate the solid and liquid phases of the sludge and to treat the two streams under different operating conditions in two separate reactors: the solid phase was fed to a thermophilic hydrolyzing reactor and the liquid phase was fed to a mesophilic high-rate digester, the Periodic ABR (PABR, Skiadas et al., 1998). The hydrolyzing reactor was operated at a high solids retention time but low hydraulic retention time. This was achieved by separating the effluent of the hydrolyzing reactor into its solid and liquid parts and then recirculating the solid part to the first stage (hydrolyser), while feeding the liquid part (rich in volatile fatty acids) to the second stage (PABR). The PABR effluent was partly recirculated to the first stage to maintain low hydraulic retention time for the hydrolyser. The overall yield of the process was 16 l methane/l wastewater at a hydraulic retention time of 19 d.

Reference

Skiadas I.V. and Lyberatos G. (1998). The periodic anaerobic baffled reactor. Wat. Sci. Technol., 38 (8-9), 401-408.
43.

MESOPHILIC ANAEROBIC DIGESTION OF WASTE ACTIVATED SLUDGE: FOCUSING ON BIOGAS PRODUCTION

David Bolzonella and Franco Cecchi

Department of Science and Technology, University of Verona, Strada Le Grazie 15, I-37134 Verona-Italy

Tel + 39 045 8027964 ; Fax + 39 045 8027928 ; [e-mail] ; [e-mail]

ABSTRACT
The need to fulfil stringent effluent standards for COD (BOD), nitrogen and phosphorous in wastewater treatments, determined in recent years the adoption of modified activated sludge processes for nutrients removal.

The biological nutrients removal processes can be performed only when the necessary amount of carbon in the treated wastewater is available so the primary settling tanks in wastewater treatment plants (WWTPs) adopting these processes are generally omitted. Moreover, in order to preserve the nitrification capability of the activated sludge process, high solids retention times (SRT) in the bioreactor are applied. Owing to this situation, a partial sludge stabilisation occurs in the oxidation-nitrification reactor. Therefore, in these WWTPs, only the waste activated sludge, which has a low biogas potential, is fed to the mesophilic anaerobic digesters: the typical specific biogas production for the waste activated sludge is in fact in the range 0.6-0.8 m³/kgVSS_destroyed rather than the typical 0.75-1.12 m³/kgVSS_destroyed of the mixed sludge (primary + secondary). This determines a decrease in biogas production so that the energetic balance of the plant is often negative, especially during winter time (Bolzonella et al., 2002). This situation is typical of a number of large wastewater treatment plants (WWTPs) worldwide.

In order to better understand this problem, four Italian large WWTPs without primary sedimentation were considered (size in the range 50.000 – 300.000 EI). Moreover, also a pilot scale Membrane Biological Reactor (MBR) was studied to get information about processes operating in extreme conditions of SRT (> 200 days). In all these WWTPs the anaerobic digestion of secondary sludge is performed in mesophilic reactors working with an organic loading rate (OLR) of some 1 kgVS/m³_reactorday and a hydraulic retention time (HRT) of some 30 days.

An inclusive set of information and data about the wastewater and waste sludge treatments and the applied operational conditions will be presented in the full paper. Table 1 reports the list and the size of the considered WWTPs.

Particular attention will be paid to the presentation of the energetic balances of these plants and to some alternatives to improve these balances as well.

Table 1- WWTPs considered in this study

Plant	Applied process	Size, EI	Flowrate, m^3/d
Fusina-Venezia	C-N	300.000	100.000
Viareggio	C-N	120.000	30.000
Treviso	BNR	70.000	12.000
San Bonifacio	C-N	50.000	10.000

C-N: pre-denitrification process; BNR: biological nutrients removal process

According to the collected data it was evident the relation between the specific gas production (SGP, m³biogas/kgVSS_fed) and the applied SRT in the activated sludge process. In particular, the highest was the applied SRT the lowest was the biogas production.

The SGP remained around 0.22 m³biogas/kgVSS_fed when SRT lower 15 days were applied in the activated sludge process, but decreased dramatically to 0.1 m³biogas/kgVSS_fed when the applied SRTs were larger than 25-30 days. Figure 1 shows the course of the SGP versus the SRT applied in the activated sludge process for the monitored WWTPs.

However, from an energetic point of view, also the best situation (i.e., a SGP of 0.23 m³biogas/kgVSS_fed), can be unsatisfactory for the energetic balance of the process if the pre-thickening of sludge fed to the anaerobic digesters is inefficient.

Figure 1. Gas production for different SRT applied in the activated sludge process at full scale

According to the collected data a model of the process will be proposed: in this model the biogas production is directly link to the waste activated sludge biodegradability. This is, on the other hand, a function of he applied SRT in the activated sludge process.
The conclusions of the work are (at moment) the following:

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