Introduction and Definitions

Sludge is a generic term for a mud-like mixture in a semiliquid state consisting of a variety of solid materials and water. Sludge is commonly encountered in natural and industrial processes. In geology and limnology, sludge normally develops on the bottom of a body of water through natural sedimentation processes from solid materials consisting of silt, clay, and sand. In environmental engineering and waste management, sludge usually refers to the solids extracted in sewage treatment. Sewage sludge consists of fecal matter, ground up food from garbage disposals, silt, sand, bits of plants, living microorganisms, especially algae and bacteria; and chemical precipitates. This article focuses on sewage sludge technologies in process and environmental engineering.

The term sludge technology refers here to: (1) sludge characterization and ecology; (2) bioactive sludges, such as activated sludge for wastewater treatment, in which sludge microorganisms are mixed thoroughly with organic substrates under conditions that stimulate biological growth using the organics as food; and (3) techniques used in sludge processing, such as settling, clarification, transportation, and dewatering. Issues involved in both activated sludge technology for wastewater treatment and sludge processing are addressed in this article.

Sludge technologies are classified by analytical approach and unit operations such as:

1. techniques for the characterization of sludge, such as the determination of sludge composition, settlement indices, sludge type classification, sludge biology and ecology;

2. multiphase fluid dynamics (hydraulics) involving solid, liquid and gas flow streams;

3. multiphase mass transfer between gases, water, and solids;

4. heat transfer, for example, in thermal drying processes;

5. population balances in screening, flotation, and floccu-lation processes;

6. chemical reactions including fermentation, anaerobic, anoxic, and aerobic reactions;

7. separations, for example, settling, classification, screening, and filtration; and

8. sludge dewatering and drying based on, among others centrifugation, various filtration methods including vacuum, pressurized, belt-press and sand-bed filters, and thermal drying processes.

Applications of process systems engineering to sludge systems can be summarized as follows:

1. steady-state and dynamic modeling with the following model forms:

(a) black-box models based on input-output measurement data;

(b) gray-box models, in which black-box and mechanistic models are mixed;

(c) lumped parameter models (LPMs) described by ordinary equations (ODEs);

(d) distributed parameter models (DPMs) described by partial differential equations (PDEs);

2. process control with the following control schemes:

(a) simple PI or PID control strategy with online tuning techniques;

(b) Linear model predictive control using black-box models;

(c) Model based control using LPMs;

(d) Model based control using DPMs;

3. monitoring and diagnosis of sludge processes; and

4. instrumentation

Sewage sludge and its utilization in wastewater treatment processes are the focus of this article. As a first step, the properties of sewage sludge are required in terms of sewage compositions, settleability measured by sludge indices, sewage-type classifications, and key biological groups in activated sludge. These form a foundation for the further analysis of wastewater treatment using activated sludge processes and sludge processing.

A complete wastewater treatment process consists of primary, secondary, and tertiary treatments. Sludge removed by primary treatment is defined as primary sludge. Similarly, we can define the secondary and tertiary sludge. The major purpose for the secondary wastewater treatment is to remove the soluble biochemical oxygen demand (BOD) that escapes primary treatment, and to provide further removal of suspended solids. The activated sludge process is a biological technique widely used for secondary wastewater treatment, in which a mixture of wastewater and biological sludge is agitated and aerated. The biological solids are subsequently separated from the treated wastewater and a part returned to the system.

Modeling, diagnosis, and control of activated sludge processes are comprehensively described in the literature. The best-known model for activated sludge processes is probably the IAWQ Activated Sludge Model No. 1 (ASM1), describing the reactions for organic carbon and nitrogen removal. The main purpose of the model is the biological reactions, while the settler dynamics are treated comparatively superficially. The ASM1 contains 13 state variables describing carbonaceous and nitrogenous removal, with each state variable having several reaction rates and stoichiometric parameters to be determined. The dynamics of phosphorus removal are addressed in the IAWQ Activated Sludge Model No. 2 (ASM2), which contains 19 states for each reactor. In spite of broad applications of ASM1, The International Water Association (IWA) (former IAWQ) Task Group identified 10 limitations of the model. Considering all these defects and more recent experimental evidence of storage of organic compounds, the task group has proposed the IWA Activated Sludge Model No. 3 (ASM3), which should correct the identified defects, and which could become a new standard for future modeling. Because of implementation time delays, ASM1 is still widely used in control studies for organic carbon and nitrogen removal.

In most activated sludge models, the classifier is treated as a simple concentrator. More structured models that incorporate both the clarification and the thickening phenomena have been presented recently. Still, the dependence of the settling parameters on the biological conditions of the sludge is not straightforward. Most of the growth models are generally described by Monod-type expressions, as are various substrate limitations and inhibitions. These are simply convenient empirical expressions without a sound theoretical foundation.

Sludge streams encountered in process and environmental engineering consist primarily of solids removed from wastewater treatment processes, and comprise a wide variety of pollutants. Furthermore, as sludge still contains more than 50% water content even after extensive dewatering operations, soluble pollutants such as ammonia and nonbio-logically degradable COD are also present. Consequently, further processing before sludge disposal is necessary.

The general sludge treatment involves concentration through thickening and floatation, stabilization of biodegradable organics, conditioning, dewatering, oxidation or incineration, and ultimate disposal of the stabilized and dewatered residues. Key unit operations used in activated sludge processes for wastewater treatment and sludge processing are presented in the following.

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