Commonly sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes.
Specific sludge production in wastewater treatment varies widely from 35 to 85 g dry solids per population equivalent per day (gTS PE-1 d-1).
The production of primary sludge is related to the amount of settleable solids in raw wastewater whose solids content is typically of 50-60 gTSS PE-1 d-1 or 110-170 gTSS/m3 of treated wastewater (Tchobanoglous et al., 2003).
Organic matter is oxidised by heterotrophic microorganisms to produce H2O and CO2 in the process known as catabolism. This process requires the availability of an electron acceptor – which may be oxygen or nitrate – and lead to the production of energy as ATP. This energy is then used by microorganisms to grow forming new cellular biomass and to guarantee maintenance functions (such as the renewal of cellular constituents, maintenance of osmotic pressure, nutrient transport, motility, etc…) in the process called anabolism.
Simultaneously biological decay of cellular biomass occurs, which creates two fractions:
- biodegradable particulate COD (XS);
- endogenous residue considered as inert particulate COD (XP), which accumulates in the system.
The XS fraction is subjected to hydrolysis process and is further oxidised to generate new cellular biomass (cryptic growth), while the endogenous residue (8-20%) remains and accumulates in the sludge.
A simplified scheme of these processes leading to sludge accumulation in a biological treatment of influent wastewater is indicated in the Figure.
Simplified scheme of the processes leading to sludge production in the biological treatment of influent wastewater.
The excess sludge produced by WWTPs with SRT of around 10-20 days can be estimated on the basis of the following typical values, in which research data (own data) produced by the authors during long-term monitoring of WWTPs located in the province of Trento (Italy) are indicated.
Specific production of excess sludge in WWTPs with and without digestion.
The sludge is commonly quantified with reference to analyses of TS, VS, TSS, VSS, total COD or particulate COD. These measurements are different because they take into account the different constituents of sludge:
1) TS (Total Solids) = quantification of solids both in soluble and in particulate form, and both organic and inorganic;
2) VS (Volatile Solids) = quantification of organic solids, both in soluble and particulate form;
3) TSS (Total Suspended Solids) = quantification of particulate solids, excluding soluble solids both organic and inorganic;
4) VSS (Volatile Suspended Solids) = quantification of particulate organic solids, excluding soluble solids and inorganic solids;
5) Total COD = chemical oxygen demand including both particulate or soluble COD;
6) Soluble COD = chemical oxygen demand of soluble compounds.
7) Particulate COD = chemical oxygen demand of particulate compounds: estimated as the difference between total COD and soluble COD.
Physical fractionation of total solids in sludge.
Simplified physico-chemical fractionation of total COD in sludge.
Total COD does not consider inorganic compounds, only organic ones and it is made up of a soluble fraction and a particulate one. A relationship exists with the VSS value, throughout the conversion factor, fcv, which typically is 1.42-1.48 mgCOD/mgVSS.
Particulate COD of sludge can be further subdivided in fractions (sludge fractionation as COD). The fractions are indicated with the symbol Xy, where X indicates the particulate form:
1) heterotrophic biomass (XBH): made up of heterotrophic bacteria involved in the biodegradation of organic matter;
2) autotrophic biomass (XBA): made up of nitrifying bacteria;
3) inert particulate COD (XI): derives from the inert particulate COD present in the influent wastewater and entering the plant. When it reaches the activated sludge stage, it is not affected by the biological treatment and accumulates in sludge;
4) endogenous residue (XP): residue of the decay process of bacterial biomass which accumulates in sludge;
5) biodegradable particulate COD (XS): the slowly biodegradable COD; in activated sludge with sufficiently long SRT this fraction is often small.
Fractionation of particulate COD of sludge.
Particulate COD of sludge is made up of the terms involved in the following sum:
Particulate COD = XI + XP + XS + XBH + XBA
Omitting the two smaller fractions, XS and XBA, the composition of activated sludge can be approximated taking into account the following terms:
Particulate COD = XI + XP + XBH
In some cases, especially when the Sludge Retention Time (SRT) in a WWTP is long, the fractions XI and XP can be greater than the fraction XBH itself.
The variations of the VSS fractions in sludge as a function of SRT are indicated in the following figure, where the calculation is based on our own data as follows:
- concentration of total COD in raw wastewater of 466 mg/L
- concentration of biodegradable COD in raw wastewater of 369 mg/L
- concentration of non biodegradable particulate COD in raw wastewater of 81 mg/L
- conventional values of stoichiometric and kinetic parameters (at temperature of 16°C) are assumed.
An example of variations of VSS fractions in sludge as a function of SRT.
In this example the heterotrophic biomass is only a limited part of the VSS of sludge, around 35% at SRT of 20 days (which corresponds to 25% of TSS of sludge). An increasing mass of the inert fractions XI and XP will accumulate in the sludge for increasing values of SRT.
The sludge fractionation is important to correctly evaluate the potential of a sludge reduction route, because to obtain high sludge reduction it is necessary to act on inert fractions which often account for the majority ofsludge mass, especially in the case of biological processes with long SRT. The conversion of inert particulates to a biodegradable substrate significantly affects sludge reduction efficiency. If this conversion process is zero in a certain technique, complete sludge reduction is obviously not possible.
With regard to sludge reduction techniques, an optimal strategy should lead to the following desirable objectives:
- to attack, solubilise or reduce the inert fractions, XI and XP, converting them into solubile or, better still, biodegradable compounds;
- to reduce the net grown biomass, but to keep the active biomass XBH high in order to ensure that the biological process remains efficient.
The material in this article is covered in the book, Sludge Reduction Technologies in Wastewater Treatment Plants, published by IWA Publishing.
Sludge Reduction Technologies in Wastewater Treatment Plants is a review of the sludge reduction techniques integrated in wastewater treatment plants with detailed chapters on the most promising and most widespread techniques. The aim of the book is to update the international community on the current status of knowledge and techniques in the field of sludge reduction. It will provide a comprehensive understanding of the following issues in sludge reduction:
principles of sludge reduction techniques;
advantages and drawbacks;
economics and energy consumption.
This book is essential reading for managers and technical staff of wastewater treatment plants as well as graduate students and post-graduate specialists.
Paola Foladori, Gianni Andreottola, Giuliano Ziglio, Sludge Reduction Technologies in Wastewater Treatment Plants, IWA Publishing: 2010, ISBN: 9781843392781
Tchobanoglous G., Burton F.L., Stensel H.D. (2003) Wastewater Engineering: Treatment and Reuse. Inc. Metcalf & Eddy, McGraw-Hill, 4th edition.
Ginestet P., Camacho P. (2007) Technical evaluation of sludge production and reduction. In: Comparative evaluation of sludge reduction routes, pp. 1-15. IWA Publishing Ltd, Londra, UK. ISBN: 1843391236.
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