Activated Sludge

The following sections are included:

• Dedication
• Preface
• Acknowledgments
• Contents

Activated sludge remains the most widely used biological wastewater treatment process throughout the world, treating both domestic and a wide range of industrial wastewater…

The following sections are included:

• Introduction
• The Microbial Growth Curve
• Rates of Reaction
• Enzyme Reactions
• Environmental Factors Affecting Growth
• Kinetic Equations of Bacterial Growth
• Modeling
• References

The following sections are included:

• Introduction
• Floc Formation
  • Introduction to Floc Formation
  • The Role of Quorum Sensing
  • The Process of Floc Development
• Floc Structure, Aging and Fragmentation
• Granular Activated Sludge
• References

To successfully operate an activated sludge reactor, the environmental conditions to which the flocs are exposed must be carefully managed. This is achieved by controlling a number of different operational parameters, which, in turn, influence the rate of reaction, growth rate of the biomass, removal efficiency of both carbonaceous matter and nutrients (N and P), and finally, microbial community characteristics. The key operational control parameters are the loading rates of wastewater to the reactor, measured as the volumetric, organic and sludge loading, sludge age or sludge residence time, oxygen or redox control, and recirculation rates. These are supported by several other parameters that monitor the biomass concentration, settleability, and activity. These are described below, but a more extensive review of loading rates is given by von Sperling et al. (2020).

The activated sludge process is affected by the nature of the wastewater being treated as well as environmental, climatic, and hydrological factors. The key factors affecting performance are temperature, which is closely linked to dissolved oxygen availability, organic loading, including the constituents of the wastewater, and hydraulic loading.

The following sections are included:

• Introduction
• Surface Aeration
  • Vertical Shaft Aerators
  • Horizontal Shaft Aerators
• Air Diffusion
  • Fine Bubble Diffusion
  • Coarse Bubble Diffusers
• Hybrid Aerators
• Aeration Control
• Testing Aerators
• References

The following sections are included:

• Introduction
• Plug-flow Systems
  • Accumulation-regeneration Theory
  • Tapered Aeration
  • Contact Stabilization
  • Incremental (Step) Feeding
  • Incremental Sludge Feeding
• Completely Mixed Systems
  • Design Basics
  • Design Parameters
• Sequencing Batch Reactor Technology
• References

In extended aeration, the sludge-loading level is very low, between 0.03 and 0.15 kg biochemical oxygen demand (BOD) kg⁻¹d⁻¹, which results in the process being food-limited, forcing the microorganisms, which comprise the activated sludge, into the endogenous respiration phase of activity (Figs. 2.2, 2.3 and 7.1). Therefore, the waste is fully oxidized (95% BOD reduction) and less sludge is produced (0.2–0.4 kg DS kg⁻¹ BOD removed), which is also more stabilized and more easily dewatered than conventional activated sludge. Organic loadings range from 0.24 to 0.32 kg BOD m⁻³d⁻¹. This type of low-rate activated sludge process has a long hydraulic retention time (HRT) of between 1 and 3 d, and as the sludge activity is very low, it requires less intensive aeration than conventional or high-rate processes. The sludge age of extended aeration processes, as expected, is quite long at >5–6 d (Table 4.1).

The following sections are included:

• Introduction
• A-B Process
• High Purity Oxygen Processes
  • Closed Systems
  • Open Systems
• Vertical Shaft Processes
• Integrated Fixed-Film Activated Sludge Systems
• References

The following sections are included:

• Membrane Biological Reactors
• Membrane Aerated Biofilm Reactors
• References

Like all biological treatment processes, the activated sludge system relies on a mixed culture of bacteria to carry out the basic oxidation of the organic material present, with higher grazing microorganisms also present, thus forming a complete ecosystem with various trophic levels. The activated sludge aeration tank is a truly aquatic environment, although the high biochemical oxygen demand (BOD) and high level of bacterial activity make it unlike any natural aquatic habitat. The high-shear environment caused by constant aeration and recirculation of sludge makes it inhospitable for most aquatic species, especially those larger than the smaller mesofauna, such as rotifers and nematodes. The main biological groups present are bacteria, fungi, protozoans, rotifers, and nematodes, with flocs a heterogeneous mixture of them all (Sec. 3.3), together with organic and inorganic material. Larger metazoan species such as Crustacea (e.g., Cyclops), annelids (e.g., Aelosoma), and even larvae of some dipterans are occasionally observed, although they are not major components of the community structure under conventional operating conditions. Algae are also present in the mixed liquor but rarely become established. In contrast to percolating filters (Gray, 2021), where the active biomass is attached to a static support medium and the wastewater flows over the surface; activated sludge is a dispersed growth system with the microbial biomass present as discrete flocs suspended in the wastewater and mixed together by the aeration system. Due to the physical difference between a static and dispersed growth system, fewer trophic levels are present in the activated sludge process (Fig. 11.1). The larger macro-invertebrate grazers associated with percolating filters and other biological treatment systems are largely absent as they cannot be supported within the floc. Thus, apart from certain protozoans and nematodes, most of the larger grazers are found swimming in the liquid phase between the flocs. A simplified food web for conventional activated sludge is given in Fig. 11.2…

Before the 1990s, identification and quantification of bacteria were very limited, based on a mixture of microscopy and culture-based methods. Our understanding of the biology of activated sludge was fundamentally changed by the development of fluorescence in situ hybridization (FISH) and epifluorescence microscopy, which allowed researchers to both identify and quantify the specific bacteria and archaea that make up the mixed liquor. The process was developed in 1970, but it was not until the 1990s that direct probe labeling was developed for rapid identification of key bacterial groups and, more recently, using PCR-generated and DNA analog probes (Huber et al., 2018). This has made FISH a major tool in enabling our understanding of the role of specific bacteria in the activated process and to help in troubleshooting sludge-based operational problems. We are now able to identify key groups involved with nitrification-denitrification, polyphosphate and glycogen accumulation, hydrolysis, and fermentation, allowing full development of BNR systems. The microorganisms responsible for more traditional operational problems such as filamentous and non-filamentous bulking, foaming, and other separation problems causing microorganisms can also be accurately identified (Chap. 17). The process is very rapid but, in practice, is done in central, research, or specialist commercial laboratories rather than at the treatment plant itself. The use of FISH in biological wastewater treatment is reviewed by Nielsen et al. (2009)…

Those fungi established in activated sludge can be loosely classified as filamentous, unicellular, or pathogenic. Filamentous fungi are large; often, branched hyphae are very characteristic and easily differentiated from the much narrower bacterial filaments. The cells walls are Gram-negative, and the fungi proliferate in activated sludge under specific operating conditions and can be diagnostic. They can outcompete bacteria when either nitrogen (N) or phosphorus (P) concentrations are limiting for bacteria growth, when the wastewater is acidic (pH<6) or when treating wastewater containing antibiotics, although some species are sensitive to antibiotics. Unicellular fungi are a common and widely distributed group of fungi associated with soils, vegetation, and a wide variety of industrial processes where yeast is used, such as in brewing, alcohol production, and baking. Yeasts are also common in fruit and vegetable processing wastewaters. This group of fungi are commonly found in activated sludge, and also widely used for bioaugmentation as they are able to rapidly degrade a wide range of organic substrates, many of which are difficult for heterotrophic bacteria to degrade. All pathogenic fungi associated with plants, humans, and livestock can also be found in wastewater but play no part in the treatment process. Two are very common: Aspergillus fumigates and Candida spp., which cause respiratory tract and oral/vaginal infections, respectively, and may present a low risk of infection to operators via skin contact or by inhalation (Sec. 20.2)…

The important role of protozoa in activated sludge has been recognized for over 90 years (Cramer, 1931; Jenkins, 1942), with their importance as indicators of sludge quality for almost as long (Arden and Lockett, 1939; Reynoldson, 1942; Pillai and Subrahmanyan, 1944). Protozoa are common components in activated sludge, with population densities reaching up to 50,000 ind. mL⁻¹, which can represent as much as 5–12% of the dry weight of the mixed liquor (Hopwood and Downing, 1965; Pike and Curds, 1971; Curds and Hawkes, 1975). This is similar to the total mass of bacteria, both viable and non-active, in the mixed liquor (HMSO, 1971); thus, protozoans can represent a major proportion of the total biomass. This was demonstrated by Aescht and Foissner (1992), who found that in terms of organism biomass, protozoa represented 4% and bacteria 96% in an activated sludge plant treating 1,400 m³d⁻¹ of pharmaceutical wastewater at a high loading rate of 21 kg BOD₅ m⁻³d⁻¹, which represented 4 tonnes (wet weight) of protozoa daily…

The largest grazers normally encountered in the activated sludge process are rotifers, nematodes, and oligochaete worms. Apart from rotifers, little is known of the specific role of the other groups in the activated sludge process, which generally only represents a minor part of the microbial biomass. However, potential positive effects include facilitating floc formation, reducing excess biomass production through the consumption of dispersed and surface bacteria, including pathogens, the consumption of suspended solids, which increases effluent clarity while reducing biochemical oxygen demand (BOD), as well as facilitating nutrient cycling in activated sludge (Chen et al., 2004; Lapinski and Tunnacliffe, 2003). All three groups require long sludge ages to become established and sustain their numbers in the mixed liquor due to relatively long life cycles.

The following sections are included:

• Nitrogen Removal Processes
• Nitrification
  • The Process of Nitrification
  • The SHARON^® Process
• Denitrification
  • The Process
  • The Denitrifying Filter
  • The Anammox^® Process
  • Anoxic Zones or Reactors
• Simultaneous Nitrification-Denitrification
• Phosphorus Removal
  • Chemical Phosphorus Removal
  • Enhanced Biological Phosphorus Removal
• Biological Nutrient Removal Systems
• Nitrous Oxide Generation in Activated Sludge
• References

The following sections are included:

• Introduction
• Foaming
• Other Non-bulking Sludge Problems
  • Deflocculation
  • Pin-point Floc
  • Denitrification
• Bulking Sludge Problems
  • Filamentous Bulking
  • Non-filamentous Bulking
• References

The following sections are included:

• Identifying Problems
• Specific Control Measures
  • Operational Control
  • Chemical Addition
  • Process Modifications
• References

Not all problems in the activated sludge process are associated with inadequate separation. Operational problems such as increased bio-chemical oxygen demand (BOD) or suspended solids in the final effluent, low mixed liquor suspended solids (MLSS) concentrations in the aeration tank, or a reduced oxygen utilization rate by the mixed liquor can all be caused by factors other than those associated with bulking. The step-by-step guide that follows should allow the operator to logically eliminate and hopefully identify and subsequently rectify non-bulking problems. It is only a guide, however, and not a full-proof key, as each activated sludge plant is so very different in design and operation, but more importantly, in the wastewater characteristics and the microbial community. It is suggested that the key is used as a starting point for operators and that it is adapted, with experience, to suit your own plant. Users should bear in mind the constraints of the design of their own plant when using the key; for example, oxidation ditches are not differentiated from conventional plants, nor are plug flow from completely mixed systems. You are advised to refer to the appropriate section in the text before making any major operational changes. The key indicates the questions that the operator should be asking; however, be prepared to try more than one of the suggested corrective measures, as the possible reasons for the problem are eliminated. The essence of a successful activated sludge operation is understanding how the process works and how the various activities that occur in the aeration tank and the final settlement tank interrelate. So the greater the understanding you have of the process, the greater the chance you have of identifying and correcting problems. The first thing to do is to check that the operational settings in the SCADA system correspond to the original design settings. Also, use the data acquisition function of SCADA to define the problem accurately and explore how it has developed over time. However, do not rely solely on the information provided by the computer. For example, check the oxygen/redox situation within aeration, anoxic or anaerobic tanks physically, checking different locations and depths…

The following sections are included:

• Environmental Problems
  • Noise
  • Aerosols
  • Odor
  • Trace Organics
  • Microplastics
• Pathogen Removal
  • Removal Mechanisms and Efficiency
  • Antibiotic Resistance
• References

The following sections are included:

• Can Activated Sludge be Sustainable?
• New Developments in Activated Sludge
  • Activated Sludge Odor Control
  • Carbon Harvesting
  • Byproduct Recovery from Waste Activated Sludge
  • Aerobic Granular Sludge
• Future Challenges
• Conclusion
• References

The following section is included:

• Index