Iwa international Specialist Conference

Déléris S. , Humbert S., Larose A. and Lebrun T.

ONDEO-Degrémont research center, 87 chemin de Ronde 78290 Croissy-sur-Seine, France

Tel : [phone] - Fax : [phone] –

e-mail :[e-mail]

introduction

Management of the excess sludge production resulting from biological wastewater treatment are one of the most important economic and environmental issues for the next decade. Due to the intensification of wastewater treatment, a large increase in sludge production from biological processes must be anticipated. Moreover, sludge agricultural valorisation and landfilling, which are the main solutions adopted for final sludge disposal, are debated and sludge incineration cannot be a systematic solution. So, new stringent regulations regarding sludge treatment and disposal imposed in several countries as well as social and environmental concerns, have resulted in an increasing interest in emergent processes aimed at the reduction or minimization of excess sludge production. Such processes propose new integrated strategies for sludge management based on the reduction of the biomass growth rate and on the intensification of sludge mineralization, both resulting from the synergy between a biological process and the application of a stress, based for example on a thermal or chemical

Following a 5 years on-going research program, Ondeo-Degrémont has developed a new set of processes designed to reduce the amount of sludge generated during biological wastewater treatment : Biolysis®. Two different versions of this sludge reduction technology are available.
The results which will be presented in the final paper will relate to two case studies aimed at assessing the effect of Biolysis® O and Biolysis® E associated with an activated sludge process. The paper will highlight results concerning reduction of sludge production and will provide a comprehensive and accurate set of results dealing with the influence of Biolysis® processes on the remaining sludge and effluent characteristics. An economical evaluation of the Biolysis® technologies will also be provided.
Ozone-activated sludge combined system, Biolysis® O : case study on a 1000 p.e. existing activated sludge plant
Biolysis® O : Sludge reduction is obtained trough the association of an ozonation treatment to a conventional activated sludge process. Ozone is directly applied on the sludge taken from the biological reactor before returning the ozonated liquor to the aerated tank. High ozone transfer is obtained using a mechanical mixing device which insures high level of reaction between sludge and ozone. A demonstration plant was run for more than one year on an 1000 p.e. existing activated sludge plant located in Aydoilles (France).

During the experimental period, the global sludge reduction efficiency was 60 %, nevertheless higher reduction efficiency (until 80 %) can be reached depending on the ozone dosage applied on the system. Slight decrease in COD removal efficiency (< 5%) was observed and SS and ammonia removal efficiencies were maintained and even improved (see table1).

Table 1 : effect of Biolysis® O on sludge production and effluent quality

Parameters	Reference results without Biolysis® O	Results with biolysis® O
Sludge production (Kg SS/ p.e/ yr)	22	8.8 (60 % reduction)
Decantability (SVI ml/g)	> 200	< 80
COD removal efficiency (%)	90	85
SS removal efficiency (%)	95	95
N-NH4 removal efficiency (%)	96	98

High enhancement in sludge settling and dewatering characteristics were also observed.

This one year testing period on Biolysis® O technology confirmed that high sludge reduction level can be reached and reveal some major advantages of the ozone activated sludge combined system :

• 
  Equipment compactness
  
• 
  Easy and fast start-up procedure
  
• 
  Limitation of bulking phenomena and filamentous bacteria development
  
• 
  enhancement of sludge settling and dewatering characteristics
  

Thermophilic enzymatic treatment-activated sludge system, Biolysis® E : case study on a 3000 p.e. existing activated sludge plant
Biolysis® E : sludge reduction process is based on the association of an conventional activated sludge process with an aerobic thermophilic solubilisation step. Sludge to be treated is taken from the biological reactor and sent to an aerated thermophilic reactor after a prior thickening stage. Conditions maintained in the thermophilic reactor allows the growth of specific bacteria (Bacillus stearo thermophilus) which have the capability to release enzymes responsible for the break down and the solubilisation of the sludge. The treated and degraded sludge is then flowed back to the sludge tank.
This technology was originally developed by the Japanese company, Shinko Pantec, which claimed that up to100 % sludge reduction is possible for some completely biological sludge.
A demonstration plant has been run for 1 year on a 3000 p.e. existing activated sludge plant located in Verberie, France. The results obtained confirm that high level of sludge reduction are reachable (until 80%) and that effluent and sludge characteristics are almost similar to those obtained with Biolysis®.

14.

PARTIAL OZONATION OF RETURN SLUDGE TO REDUCE EXCESS SLUDGE, IMPROVE DENITRIFICATION AND CONTROL SCUMMING

Hansruedi Siegrist and Marc Boehler
EAWAG, Swiss Federal Institute for Environmental Science and Technology

Ueberlandstr. 133, PO Box 611, CH-8600 Duebendorf, Switzerland
tel. [phone], fax [phone], email: [e-mail]

KEYWORDS
Ozonation, activated sludge, excess sludge reduction, nitrification, denitrification, bulking, scumming,

EXTENDED ABSTRACT
Agricultural use of stabilized sewage sludge will be banned in Switzerland in 2005 due to ecotoxicological considerations (heavy metals and trace pollutants). Because disposal in landfills is already forbidden in Switzerland, sewage sludge has to be dewatered, dried, incinerated and the ashes disposed in landfills. These processes are cost intensive lead also to the loss of valuable phosphate resources incorporated in the sludge ash. The implementation of processes that could reduce excess sludge production and recycle phosphate is therefore recommended.
Yasum et al. (1994 and 1998) describe a significant decrease of excess sludge production in a pilot plant fed with artificial wastewater (yeast extract and peptone) by daily treatment of a fraction of the reactor sludge with ozone. With an optimum ozone dosage of 0.05 gO₃ gSS^-1 and aeration of the ozonated sludge suspended solids concentration was reduced by 30%. A higher ozone dosage did not yield a significant higher sludge reduction. Below 0.05 gO₃ gSS^-1 suspended solids concentration decreased proportionally to the ozone application. The laboratory plant was operated with a solid retention time (SRT) of 10 days, a suspended solids concentration of 4000 gSS m^-3 and a BOD load of 1000 gBOD m^-3 d^-1. The biomass yield without ozone application was 0.4 gSS gBOD^-1, corresponding to 400 gSS m^-3 d^-1. Excess sludge production decreased proportional to the daily portion of activated sludge treated with ozone. By treating daily 30% of the activated sludge no excess sludge was produced. This corresponds to an ozone application of 4000·0.3·0.05/400 = 0.15 gO₃ gSS_{excess sludge}^-1 or 0.06 gO₃ gBOD_inlet^-1.
Ried et al. (2002) describe a two-lane full-scale activated sludge plant treating the wastewater of 16’000 p.e. with a solid retention time of 15 days. In one lane a portion of the return flow was ozonated. They measured a nearly 30% reduction of the excess sludge production in the ozonated lane by treating daily 10% of the activated sludge with an ozone dosage of 0.052 gO₃ gSS^-1. This corresponds to an ozone dosage of about 0.08 gO₃/gSS excess sludge. The result with real wastewater is similar to the above described laboratory plant.
Ried et al. (2002) observed a 30-40% reduction of the sludge volume index in the ozonated lane and a substantial improvement of excess sludge dewatering.
Because the optimum ozone dosage is low (0.05 gO₃ gSS^-1) at least 10% of all activated sludge have to be treated daily to significantly reduce excess sludge production. The effective SRT of the nitrifiers could be strongly reduced due to inactivation of nitrifiers by ozone. On the otherhand the nitrifiers might be protected inside the activated sludge floc (the faster growing heterotrophs overgrow the slower growing nitrifiers). In our laboratory plant and batch reactor experiments (Boehler and Siegrist, 2002) we would like to confirm the above results with real wastewater and investigate the effect of ozone treatment on nitrifiers.
If excess sludge production and SVI are significantly reduced by ozonation without inhibiting nitrification the activated sludge and secondary clarifier volume can be reduced. Ozonation of activated sludge could therefore be an economical process for overloaded plants. Less excess sludge production reduces also raw sludge load of sludge digestion and could prevent the enlargement of overloaded digesters.
Additionally we will investigate the improvement of denitrification capacity due to readily degradable COD produced by ozonation. Yasum et al. (1994) and Ried et al. (2002) describe both a solubilization of the ozonated activated sludge of about 3 gCOD gO₃^-1. Other aspects of our investigation are the effect of ozonation on potential scumming and bulking of activated sludge and P-recycling.
With the reduction of excess sludge production the phosphate fraction incorporated in the excess sludge will again dissolve and increase P concentration in the effluent. Ozonation could therefore be an interesting process to recycle the valuable phosphate of the activated sludge by post precipitation.
Based on the experiments of Yasum et al. (1994) and Ried et al. (2002) the increase of the energy consumption due to ozonation was estimated. Based on a nitrifying-denitrifying plant with an average SRT of 15 days energy consumption increases by 5-10% for an ozone dosage of 0.08 gO₃ gSS^-1 excess sludge (15-20 kWh/kg O₃ produced from air) and excess sludge production is reduced by 30%. If ozonated sludge is directly added to the sludge digester digested sludge is also reduced and biogas production significantly increased, which compensates the energy input for ozonation (Vranitzky et al., 2002).
Cost for operation and investment of a partial sludge ozonation is compensated by the decreasing operation cost for sludge treatment and disposal (Boehler and Siegrist, 2002) . The pay off periode of sludge ozonation is strongly reduced if an enlargment of a plant could be delayed or prevented.

REFERENCES

Böhler M. and Siegrist H. (2002) Reduktion des Belebtschlamms und Bekämpfung von Blähschlamm durch Ozonierung, Conference printings, Reach, Basel, Switzerland, October.

Ried, A.,Stapel H., Koll R. Schettlinger M., Wemhöner F., Hamann-Steinmeier A., Miethe M., Brombach A. (2002) Optimierungsmöglichkeiten beim Betrieb von biologischen Kläranlagen durch den Einsatz von Ozon, Korrespondenz Abwasser, 49 (5), 648 – 661.

Vranitzky, R., Lahnsteiner, J., Fuchs, W., Braun, R. (2002) Final Report for THE WELTBANK, Second Phase Sludge Reduction Study.

Yasui H., Shibata M. (1994) An innovate approach to reduce excess sludge production in the activated sludge process, Wat. Sci. and Tech., 30 (9), 11-20.

Yasui H., Nakamura K., Sakuma S., Iwasaki M. and Sakai Y. (1996) A full-scale operation of a novel activated sludge process without excess sludge production, Wat. Sci. and Tech., 34 (3-4), 395-404.