Iwa international Specialist Conference

Aalborg Sep. 230902

Sludge EPS Characterization

Kristian Keiding, Lisbeth Wybrandt and Per H. Nielsen

Aalborg University, Department of Environmental Engineering

Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark

Abstract

The characterization of sludge is inevitably based on the conception of the dewatering physical chemistry. Some cherish the notion that sludge is a micro gel particle system. [LeGrand et al., Water Research 32(12) 1998]. The particles, being the sludge flocs, do primarily consist of a condensed EPS-phase, in which single cells and bacterial colonies are embedded. Hence, the process of deswelling of a charged gel mirrors the sludge structure and the dewatering of this.

The structure of a charged gel is partly defined by properties, which leads to swelling and subsequently dissolution of the gel. These properties are defined by the free charge of the gel components and of the ionic environment and of the molar weight distribution of the gel components. Further, cohesive effects, defined by hydrophobic properties and charge density effects, define the structure.

The present paper addresses the analytical techniques to assess these features of the charged gel. These techniques are: ^a) colloidal- and pH-titration for the determination of the free charge, ^b) measurements of contact angle and IR-spectrophotometry for a relative assessment of hydrophobic properties and ^c) size exclusion chromatography for the assessment of molecular size distribution.

The experience in applying these techniques reveal a picture of sludge EPS as having a free surface charge density of 1-3 meq/g, as having fairly uniform hydrophobic properties and as apparently having a relatively broad and varying size distribution. The suggestion from this study is thus that a key parameter for the condensation of sludge EPS into gel particles is the molecular size distribution of the EPS. This will be illustrated from the analysis of sludge from various aerobic and anaerobic treatments.

10
HOW TO REACH ZERO EXCESS SLUDGE PRODUCTION IN THE WASTEWATER TREATMENT LINE?

Mostapha Salhi¹, Stéphane Déléris¹, Philippe Ginestet², Hubert Debellefontaine¹ and Etienne Paul^1*
(1). Laboratory of Environmental Processes Engineering, Department of Industrial Processes Engineering, National Institute for Applied Sciences, INSA – GPI – LIPE,
135, Avenue de Rangueil, 31077 Toulouse Cedex 4, France.

(2). CIRSEE, ONDEO Services, 38, Rue du Président Wilson, 78230 Le Pecq, France.

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  Tel: [phone] ; Fax: [phone]; e-mail: [e-mail]
  

In Europe, major ways of disposing of excess sludge are subject to more and more legal and social constraints, and disposal of excess biomass may account for up to 60% of total plant operating costs (Horan, 1990). Therefore, reducing excess sludge production instead of merely treating it appears to be a very appealing solution to this social and economic issue, because the problem would be treated at its roots.

However many questions arise when looking for applying a technology of reduction of Excess Sludge Production (ESP) on the treatment line. What is the strategy to follow? Which technology is the one to apply? What would be the consequences on the global process efficiency? …. This paper will answer to some of these questions.

1) What sludge material should be targeted?

Sludge production is a result of biomass growth on wastewater biodegradable fraction and refractory organic and inorganic material accumulation coming both from the wastewater and from biological and physical chemical process occurring in the reactor. So a first question is asked: among those phenomena, which one should be mainly targeted to reach a significant reduction of ESP while minimizing costs? Should biomass net growth be decreased by increasing decay rate or maintenance energy requirements? Would it be better to target the refractory solid material? In this study, these different strategies are assessed. Firstly, Activated Sludge Model 1 (ASM1) was successfully calibrated using results from lab scale pilot plants operated at different sludge retention times (SRT). Secondly, it was used to compare the maximum reduction of ESP reachable by increasing biomass decay or maintenance energy requirements or by increasing solubilisation and/or biodegradability of refractory materials. In the case of an activated sludge system operated at SRT of 15 days and fed with a primary settled wastewater containing a solid refractory COD fraction (X_I) of 20%, ASM1-based simulations demonstrated that:

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  a 20% maximum reduction of ESP could be achieved when doubling biomass decay rate
  
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  a 35% maximum reduction of ESP could be reached when increasing maintenance energy requirements simulated by decreasing the heterotrophic growth yield by 30%
  
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  increasing solubilisation and biodegradability for solid refractory materials, 100% reduction of ESP could be reached.
  

It is therefore clear that the main target for technologies applied for reduction of ESP should be the organic refractory solid material accumulating. In addition, to prevent mineral accumulation inorganic and refractory organic solid fractions should be solubilised at a same rate.
2) Then, what technologies for reduction of ESP?

Based on these results, oxidation technologies appear to be promising candidates for high reduction of ESP. Chemical oxidation may act complementary to biological oxidation because of specific affinity with unsaturated complex molecules, which are important part of particulate refractory COD fraction. Using ozone as oxidant in discontinuous reactor (2 L), solubilisation of particulate COD and mineral was followed for sludges from different origins. The high potential of ozone for solubilising both organic and inorganic fractions was then confirmed. A lab scale (26 L) activated sludge reactor continuously fed with a real primary settled wastewater from Toulouse was coupled with reactor (2 L) discontinuously fed with the mixed liquor. This discontinuous reactor served as contactor between gaseous ozone and sludge. For ozone dosage of 0.02 g O3 . (g VSS)^-1 no excess sludge production was observed (figure 1). This result confirms previous results obtained at various scales by Kamiya and Hirotsuji (1998) or Déléris (2001). So far, and from our knowledge, no other technology (thermal, mechanical, chemical, physical associated treatments) have demonstrated such a potential on real wastewater. This may be due to inadequate solubilisation potential of these last techniques.

3) What is the importance of solubilisation processes?

COD solubilisation: during reduction of ESP experiments, lost of colloïdal and particulate matter in the outlet of the activated sludge reactor is observed. For experiments showing 100% reduction of ESP, total COD or filtrable COD in the outlet are twice the corresponding values obtained for the control line. This COD lost does not account for more than 20% of the reduction of ESP, and may represent organic matter either totally refractory to ozone action or hydrolysable only within retention times higher than the one applied.

Mineral matter solubilisation: it is evident that to reach a no ESP point, the solid non-volatile fraction must be solubilised. In the continuous pilot associating ozone and activated sludge treatments, the VSS/SS ratio first decreases from 0.85 to 0.70 during first weeks of ozonation but then stabilizes at this last value. In figure 2, mass balances on solid non-volatile matter are presented for various degrees of reduction of ESP. For 0% reduction of ESP, 80% of the solid non-volatile matter from the inlet is recovered as solids in the outlet. This percentage greatly decreases with the degree of reduction of ESP until about 30% for 100% reduction of ESP, demonstrating effective mineral solubilisation during the treatment. This threshold should have represented a limitation for the reduction of ESP. In fact, the lost of particulate matter observed in the outlet of the activated sludge reactor enables elimination of non-solubilised mineral matter. In a case where all particulate lost could be prevented, one can expect either a long term solubilisation action of ozone on mineral sludge or an accumulation of mineral sludge into the reactor, even for condition where 100% reduction of organic ESP is reached because of the existence of a mineral fraction totally refractory to the solubilising action of ozone.
4) What about nutrients?

Nitrogen and phosphorus compounds are also solubilised during ozonation. In the continuous pilot 90%± 5% of nitrificable nitrogen is nitrified whatever reduction of ESP degree. Nitrification potential then increases proportionally to the nitrogen solubilisation. Moreover, aerobic and anaerobic respirometric measurements show that the COD released after ozonation is partly easily biodegradable and can be consumed for denitrification. But, remaining O₂ will compete with nitrates as final electrons acceptors.

Solubilisation of nutrients and decrease of sludge production set the problem of nutrient recovery mainly for P recovery. New strategies should be developed.

References:

Déléris S., Paul E., Debellefontaine H., Geaugey V. (1999) Sludge reduction in wastewater treatment plants. Mass balances for assessing the efficiency of a combined treatment system: activated sludge and ozonation. 2eme Congrès Européen de Génie des Procédés, Montpellier, 5-7 octobre 1999.

Kamiya T. and Hirotsuji, J. (1998) New combined system of biological process and intermittent ozonation for advanced wastewater treatment. Wat. Sci. Tech. 38 (8-9), 145-153.