Wastewater Treatment, Sludge Management and Reuse

Introduction and Context

Wastewater treatment is the controlled transformation of domestic, industrial and urban drainage flows into effluents and residual solids that can be safely discharged, reused or recovered. Modern environmental engineering no longer regards wastewater only as a waste requiring disposal. It is also a source of water, nutrients, organic matter and energy. The engineering challenge is therefore to achieve public-health protection and regulatory compliance while minimizing energy use, greenhouse-gas emissions, chemical consumption and residual waste.

A treatment system must be designed as an integrated process train. Preliminary treatment protects downstream equipment; primary treatment removes settleable and floatable matter; biological treatment converts dissolved and colloidal organics and may remove nitrogen and phosphorus; tertiary treatment polishes the effluent; disinfection controls pathogens; and sludge treatment stabilizes and concentrates the residual solids. Reuse requires an additional risk-based assessment of treatment reliability, exposure pathways and end-use quality.

The appropriate design depends on wastewater characteristics, flow variation, temperature, land availability, receiving-water sensitivity, energy costs, operator capacity and the intended end use of treated water. Process selection should therefore be based on mass balances, kinetics, reactor models, life-cycle considerations and local discharge or reuse requirements rather than on a single universal treatment sequence.

Learning Objectives

After completing this chapter, the reader should be able to:

1. Characterize wastewater flows and pollutant loads for treatment design.

2. Explain the purpose and design basis of preliminary, primary, biological and tertiary processes.

3. Apply process kinetics and idealized reactor models to biological treatment.

4. Design or check activated-sludge systems using solids-retention-time and substrate-removal concepts.

5. Explain nitrification, denitrification and enhanced biological phosphorus removal.

6. Evaluate disinfection, sludge thickening, stabilization, dewatering and disposal or reuse options.

7. Develop a risk-based approach to water reuse and resource recovery.

8. Critically assess operational, regulatory, ethical and sustainability issues.

1. Wastewater Characteristics and Design Basis

1.1 Flow and load

Treatment units are governed by both hydraulic flow and pollutant mass. The mass load of a constituent is

\[

L=QC

\]

where \(L\) is mass loading, \(Q\) is flow and \(C\) is concentration, using consistent units. For example, if \(Q\) is in m³/d and \(C\) in mg/L, then

\[

L\;(\text{kg/d})=\frac{QC}{1000}.

\]

Design should consider average daily flow, maximum-month flow, peak daily flow and instantaneous peak flow. Equalization may be justified where industrial discharges or storm-related inflows produce large fluctuations. Infiltration and inflow can dilute wastewater while simultaneously increasing hydraulic loading and pumping costs.

1.2 Major pollutants

Important wastewater parameters include total suspended solids (TSS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), ammonia, nitrate, total phosphorus, oils and grease, pathogens, salinity, toxic compounds and micropollutants.

BOD is an operational measure of biodegradable oxygen demand, commonly determined as five-day BOD at 20°C. COD measures chemically oxidizable material and is generally higher than BOD. The biodegradable fraction of COD is especially important for biological design. A wastewater with a low readily biodegradable fraction may require hydrolysis, fermentation or chemical treatment to support nutrient removal.

Nitrogen occurs as organic nitrogen, ammonia, nitrite and nitrate. Phosphorus occurs as orthophosphate, condensed phosphate and organic phosphorus. Their control is essential because discharge to sensitive waters can cause eutrophication.

1.3 Sampling and data reliability

A design data set should include flow-proportional composite samples, peak-period samples and, where relevant, industrial source characterization. Sampling frequency must reflect variability. A single grab sample cannot reliably define a wastewater for a major treatment investment. Data should be screened for outliers, analytical uncertainty, seasonal effects and changes in industrial activity. Mass balances should be performed on both average and critical design conditions.

2. Treatment Train and Process Selection

A conventional municipal treatment train is:

screening → grit removal → flow measurement/equalization → primary clarification → biological treatment → secondary clarification → tertiary polishing → disinfection → discharge or reuse.

Sludge from primary and secondary processes is thickened, stabilized, dewatered and beneficially used or disposed of. Sidestreams from sludge dewatering may contain high ammonia and phosphorus loads and should be included in the plant mass balance.

Process selection should compare performance, reliability, energy demand, chemical use, land requirement, operator skill, resilience to shock loads, residuals production and whole-life cost. A process that meets an average effluent limit but fails during wet-weather peaks is not a reliable design.

3. Preliminary and Primary Treatment

3.1 Screening

Screens remove rags, plastics, wood and other large debris. Coarse screens protect pumps, while fine screens can remove a significant portion of suspended solids before biological treatment. Design considerations include approach velocity, head loss, screen opening, cleaning method, bypass arrangements and screenings handling. Screenings are normally washed, compacted and disposed of under applicable solid-waste requirements.

3.2 Grit removal

Grit consists mainly of sand, gravel, glass and other dense inorganic particles. It causes abrasion, pipe deposition and loss of effective volume in tanks and digesters. Constant-velocity aerated or vortex grit chambers are commonly used. The target is to remove dense particles while allowing most organic suspended solids to remain in the wastewater.

3.3 Primary sedimentation

Primary clarifiers remove settleable solids and floatable materials. For an ideal clarification basin, the surface overflow rate is

\[

V_o=\frac{Q}{A_s},

\]

where \(V_o\) is surface overflow rate, \(Q\) is design flow and \(A_s\) is plan surface area. The equation is a hydraulic design criterion, not a complete prediction of removal, because actual performance depends on particle settling, short-circuiting, inlet and outlet design, temperature and sludge withdrawal.

Hydraulic retention time is

\[

\theta=\frac{V}{Q},

\]

where \(V\) is tank volume. Retention time alone does not guarantee clarification. Settling-column tests and solids-flux analysis are preferable when high reliability is required.

Primary treatment commonly removes approximately 50–70% of TSS and 25–40% of BOD, although actual values vary considerably. Enhanced primary treatment using coagulants can increase removal but produces more chemical sludge and may reduce the readily biodegradable carbon available for denitrification.

4. Biological Treatment: Concepts and Kinetics

4.1 Microbial conversion

Biological treatment uses microbial communities to oxidize organic matter and transform nutrients. Heterotrophic organisms use biodegradable organic matter as a carbon and energy source. Nitrifying organisms oxidize ammonia first to nitrite and then to nitrate. Denitrifying organisms reduce nitrate to nitrogen gas under anoxic conditions using an electron donor, usually biodegradable organic carbon.

A simplified heterotrophic growth relationship is the Monod equation:

\[

\mu=\mu_{\max}\frac{S}{K_s+S},

\]

where \(\mu\) is the specific growth rate, \(\mu_{\max}\) is the maximum specific growth rate, \(S\) is the limiting substrate concentration and \(K_s\) is the half-saturation constant. The equation assumes that one substrate is limiting and that temperature, toxicity and other environmental factors are not explicitly represented. It should not be interpreted as a complete plant model.

A commonly used temperature correction is

\[

k_T=k_{20}\theta^{(T-20)},

\]

where \(k_T\) is the rate at temperature \(T\,^{\circ}\text{C}\), \(k_{20}\) is the rate at 20°C and \(\theta\) is an empirical temperature coefficient. The value of \(\theta\) depends on the process and must be established from reliable data; it is not a universal constant.

4.2 Reactor models

In an ideal plug-flow reactor, wastewater moves through the basin with limited longitudinal mixing. In an ideal completely mixed flow reactor (CMFR), the contents are uniformly mixed and the effluent concentration equals the tank concentration. Real tanks lie between these idealizations because of dispersion, recirculation, inlet effects and dead zones.

For a first-order reaction in a plug-flow reactor:

\[

\frac{S}{S_0}=e^{-k\theta}.

\]

For an ideal CMFR:

\[

\frac{S}{S_0}=\frac{1}{1+k\theta}.

\]

Here \(S_0\) and \(S\) are influent and effluent substrate concentrations, \(k\) is a first-order rate constant and \(\theta\) is hydraulic retention time. These equations apply only when the reaction is adequately represented as first order and the reactors approach the stated ideal conditions. They are useful for comparison but are insufficient by themselves for detailed activated-sludge design.

4.3 Activated-sludge design variables

The food-to-microorganism ratio is commonly expressed as

\[

F/M=\frac{QL_0}{VX},

\]

where \(Q\) is flow, \(L_0\) is influent biodegradable substrate concentration, \(V\) is aeration volume and \(X\) is biomass concentration. Units must be checked carefully; if concentrations are in kg/m³, the result has units of inverse time.

Solids retention time (SRT), also called sludge age, is

\[

\theta_c=\frac{\text{mass of solids in system}}{\text{mass of solids wasted per day}}.

\]

For a simplified system with negligible solids in the effluent,

\[

\theta_c\approx\frac{VX}{Q_wX_w},

\]

where \(Q_w\) and \(X_w\) are waste-sludge flow and concentration. SRT strongly affects biomass inventory, nitrification, oxygen demand and sludge production. Nitrifiers grow more slowly than ordinary heterotrophs and therefore require a longer effective SRT, especially at low temperature.

The oxygen requirement includes carbon oxidation, endogenous respiration and nitrification. A conceptual design should distinguish oxygen transferred to the mixed liquor from blower energy, because oxygen-transfer efficiency changes with depth, temperature, salinity, fouling and diffuser condition.

4.4 Secondary clarification

The secondary clarifier separates biological solids from treated water and returns a controlled fraction as return activated sludge. Its performance depends on settling properties, solids loading, hydraulic loading, sludge blanket depth and denitrification within the clarifier. Rising sludge may result from denitrification, while poor settling may result from filamentous organisms, inadequate selector conditions or toxic shocks.

Clarifier design should use both hydraulic criteria and solids-flux concepts. A high return-sludge rate does not automatically correct poor settling; it may increase hydraulic loading and worsen performance.

5. Nitrogen and Phosphorus Removal

5.1 Nitrification and denitrification

Nitrification is an aerobic autotrophic process. Ammonia is oxidized to nitrite and then nitrate. It consumes oxygen and alkalinity. If alkalinity is insufficient, pH can fall and nitrification can deteriorate.

Denitrification is an anoxic reduction of nitrate to nitrogen gas. It requires nitrate, anoxic conditions, active denitrifiers and an electron donor. Internal recycle from an aerobic zone to an anoxic zone is used to return nitrate. The achievable removal is constrained by recycle ratio, carbon availability, nitrate production and mixing.

A typical biological nitrogen-removal configuration includes an anoxic zone followed by an aerobic zone, with return activated sludge and internal nitrate recycle. Post-anoxic systems may require an external carbon source when the wastewater lacks biodegradable carbon. Carbon addition improves control but increases chemical cost and residual solids.

5.2 Enhanced biological phosphorus removal

Enhanced biological phosphorus removal (EBPR) uses phosphorus-accumulating organisms exposed sequentially to anaerobic and aerobic or anoxic conditions. In the anaerobic zone, readily biodegradable carbon supports release of stored phosphate and uptake of volatile fatty acids. In the subsequent aerobic or anoxic zone, organisms take up phosphorus in excess of their growth requirement. Phosphorus leaves the system in wasted sludge.

EBPR is sensitive to carbon type, nitrate leakage into the anaerobic zone, temperature, SRT and competing microbial populations. Chemical precipitation with alum, ferric salts or lime provides an alternative or polishing step, but increases chemical sludge and may affect dewatering.

6. Tertiary Treatment and Disinfection

Tertiary processes are selected according to the residual risk and reuse objective. Common processes include coagulation–flocculation and filtration for particulate removal, activated carbon for selected organic micropollutants, membranes for dissolved and particulate separation, and advanced oxidation for recalcitrant contaminants.

6.1 Filtration and membranes

Granular-media filtration can remove residual suspended solids and improve disinfection. Membrane bioreactors combine biological treatment with membrane separation and produce a high-quality, low-TSS effluent, but require energy, membrane cleaning and management of fouling. Reverse osmosis can remove salts and many dissolved contaminants but generates a concentrate stream that requires responsible management.

6.2 Disinfection

Disinfection reduces viable pathogens; it does not necessarily remove all contaminants. Chlorination is effective and relatively inexpensive but can form disinfection by-products and requires residual management. Ultraviolet disinfection avoids a chemical residual but depends on low turbidity, adequate UV dose, lamp maintenance and reliable power. Ozone can provide strong oxidation and disinfection but has higher complexity and energy demand.

For a first-order approximation of disinfectant contact, the Chick–Watson relationship is

\[

\ln\left(\frac{N}{N_0}\right)=-kC^n t,

\]

where \(N_0\) and \(N\) are initial and surviving organism concentrations, \(C\) is disinfectant concentration, \(t\) is contact time, and \(k\) and \(n\) are empirical parameters. The equation assumes controlled conditions and does not replace validation under actual pH, temperature, turbidity, organic-demand and hydraulic conditions. Baffling and tracer testing are important because nominal detention time may substantially exceed effective contact time.

7. Sludge Management

7.1 Sludge characteristics

Primary sludge is relatively dense and readily putrescible. Waste activated sludge contains more biological solids and is generally more difficult to thicken. Sludge management should be designed from a solids mass balance, including primary solids, biological yield, chemical precipitates and sidestream returns.

7.2 Thickening and dewatering

Thickening reduces volume before stabilization. Gravity thickeners are often suited to primary sludge, while dissolved-air flotation can be effective for waste activated sludge. Dewatering technologies include centrifuges, belt filter presses, screw presses and drying beds. Polymer selection and dose should be established by jar tests or pilot testing because overdosing can increase cost and impair cake quality.

7.3 Stabilization

Anaerobic digestion converts biodegradable organic solids into methane-rich biogas and stabilized biosolids. It offers energy recovery but requires reliable heating, mixing, feed control and gas safety systems. Aerobic digestion is simpler for smaller plants but consumes oxygen and generally offers less energy recovery.

Composting uses aerobic biological heat generation to produce a stabilized soil amendment when feedstock quality, pathogen reduction and vector attraction requirements are met. Lime stabilization raises pH and reduces pathogen viability but adds material and does not provide the same degree of organic stabilization as digestion.

A simplified anaerobic-digestion methane estimate is based on the biodegradable volatile-solids destroyed and a measured or accepted methane yield. Such estimates must account for substrate composition, temperature, retention time, inhibition and actual gas losses; theoretical gas yields should not be treated as guaranteed production.

7.4 Final use or disposal

Options include agricultural land application, compost products, thermal drying, incineration, landfill and co-disposal. Land application can recycle nitrogen and phosphorus but requires control of pathogens, heavy metals, persistent chemicals, application rates, runoff and public exposure. Regulations may distinguish between treatment classes and end uses. Trace organic contaminants and per- and polyfluoroalkyl substances are increasingly important in sludge governance.

8. Water Reuse and Resource Recovery

Reuse planning begins with the end use, not with a generic treatment technology. Possible uses include agricultural irrigation, landscape irrigation, industrial cooling, toilet flushing, environmental augmentation, groundwater recharge and potable reuse. Each use has distinct requirements for pathogens, nutrients, salts, chemicals, reliability and monitoring.

A risk-based reuse framework identifies hazards, exposure pathways, critical control points, performance targets, monitoring requirements, failure response and public communication. Treatment barriers should be independent enough that failure of one barrier does not immediately create unacceptable risk. Reliability includes standby equipment, automatic shutdown or diversion, storage, alarms, validated treatment performance and competent operators.

Nutrient recovery may include struvite crystallization, ammonia recovery, phosphorus precipitation and biosolids recycling. Struvite recovery can reduce scaling in pipes and return phosphorus and magnesium as a fertilizer, but it requires suitable chemistry and markets. Biogas can be upgraded to biomethane, while heat can be recovered from effluent or engine exhaust.

The environmental value of recovery must be evaluated using life-cycle assessment. A process that recovers phosphorus but consumes excessive chemicals or produces hazardous residues may not be preferable to a simpler alternative.

9. Illustrative Integrated Design Case

Background

This fictional case concerns a municipal plant serving 50,000 people. The average flow is 10,000 m³/d, peak daily flow is 18,000 m³/d, influent BOD is 250 mg/L, TSS is 300 mg/L, and ammonia-nitrogen is 35 mg/L. The receiving river is sensitive to ammonia and eutrophication. The municipality also intends to reuse part of the effluent for landscape irrigation.

Problem definition

The plant must provide reliable carbon removal, nitrification, nitrogen reduction, phosphorus control, pathogen reduction and a reuse-quality effluent. The existing plant has primary clarification and conventional aeration but experiences ammonia breakthrough in winter and poor clarifier settling during wet weather.

Preliminary load calculation

Average BOD load is

\[

L_{BOD}=\frac{(10,000)(250)}{1000}=2,500\text{ kg/d}.

\]

Average TSS load is

\[

L_{TSS}=\frac{(10,000)(300)}{1000}=3,000\text{ kg/d}.

\]

Average ammonia-nitrogen load is

\[

L_{NH_3-N}=\frac{(10,000)(35)}{1000}=350\text{ kg/d}.

\]

These are average loads; equipment must also be checked at peak hydraulic and pollutant conditions.

Approach

The proposed upgrade includes improved screening and vortex grit removal, primary-clarifier rehabilitation, an anoxic–aerobic activated-sludge system with internal nitrate recycle, improved secondary clarification, chemical phosphorus polishing, tertiary filtration and UV disinfection. Waste activated sludge is thickened and co-digested with primary sludge in an anaerobic digester.

For an initial biological-volume estimate, suppose the design selects an average mixed-liquor concentration of 3.0 kg/m³ and a target F/M of 0.15 d⁻¹ based on pilot and full-scale experience. If the biological influent BOD load after primary treatment is estimated at 1,625 kg/d, then

\[

V=\frac{L_0}{(F/M)X}

=\frac{1,625}{(0.15)(3.0)}

=3,611\text{ m}^3.

\]

This is an initial sizing value, not a final design. It must be checked for winter nitrification SRT, oxygen-transfer capacity, anoxic volume, alkalinity, clarifier solids flux, peak flow and sludge production.

Outcome and learning points

Pilot testing and winter commissioning show that the longer SRT prevents ammonia breakthrough, while internal recycle and controlled carbon distribution improve total-nitrogen removal. Tertiary filtration reduces turbidity and improves UV reliability. Digestion reduces volatile solids and offsets part of plant electricity demand through biogas utilization. The project also establishes automatic diversion of non-compliant effluent from the reuse line.

The case demonstrates that treatment design is a system problem. Increasing aeration alone would not solve clarifier limitations, carbon deficiency, disinfection reliability or sludge management. The final solution integrates process kinetics, hydraulics, operational control and risk management.

10. Control, Monitoring and Professional Practice

Online sensors for dissolved oxygen, oxidation–reduction potential, ammonia, nitrate, turbidity, UV intensity, flow and sludge blanket depth can improve control, but sensors require calibration, cleaning and verification. Supervisory control and data acquisition systems support trend analysis and alarms; they do not replace operator judgment.

Artificial intelligence can assist with aeration optimization, fault detection, influent forecasting and predictive maintenance. However, models may fail under unusual industrial discharges, sensor drift or changing microbial populations. AI should therefore be treated as decision support with human oversight, documented validation, cybersecurity protection and an auditable fallback mode.

Professional responsibility includes accurate sampling, transparent reporting, prevention of unauthorized bypasses, protection of workers from pathogens and toxic gases, and equitable consideration of communities affected by discharge or reuse schemes. Reuse decisions must not transfer risk to less powerful users or agricultural workers. Public communication should explain both benefits and residual risks without overstating treatment performance.

11. Challenges and Contemporary Developments

Major challenges include emerging contaminants, antimicrobial-resistance genes, climate-driven flow and temperature changes, energy price volatility, salinity accumulation, drought, aging infrastructure and shortages of skilled operators. Decentralized and nature-based systems may be appropriate where land and maintenance conditions permit, but they still require reliable pathogen control and monitoring.

Energy-positive plants increasingly combine high-rate biological treatment, anaerobic digestion, biogas upgrading, heat recovery and nutrient recovery. Digital twins can support scenario analysis, but their value depends on representative data and calibrated process models. Advanced oxidation and high-pressure membranes can produce excellent effluent quality, yet concentrate management and energy demand remain significant limitations.

Best practice is to establish treatment objectives from receiving-water and reuse risks; use conservative but evidence-based design assumptions; validate critical barriers; maintain redundancy; design for maintainability; monitor mass balances; and review performance under actual seasonal and peak conditions.

Chapter Summary

Wastewater treatment is an integrated sequence of hydraulic, physical, chemical and biological processes. Preliminary treatment protects equipment, primary clarification removes settleable solids, biological reactors remove organic matter and nutrients, tertiary processes polish the effluent, and disinfection reduces pathogen risk. Kinetic relationships such as the Monod equation and reactor models provide conceptual and preliminary design tools, but full-scale design must account for temperature, solids retention time, oxygen transfer, settling, recycle and process interactions.

Nitrogen removal requires coordinated nitrification and denitrification, while phosphorus may be removed biologically, chemically or through combined systems. Sludge must be thickened, stabilized, dewatered and responsibly reused or disposed of. Anaerobic digestion and nutrient recovery can convert residuals into energy and products. Water reuse must be based on intended use, multiple treatment barriers, validated performance and risk management. Reliable practice combines quantitative design, operational resilience, regulatory compliance, ethical governance and life-cycle evaluation.

Key Terms

BOD: Biochemical oxygen demand measured under specified test conditions.

COD: Chemical oxygen demand representing chemically oxidizable material.

TSS: Total suspended solids present in wastewater.

Surface overflow rate: Flow divided by clarifier plan surface area.

Hydraulic retention time: Reactor volume divided by flow.

Monod equation: Saturation relationship between microbial growth rate and substrate concentration.

F/M ratio: Food-to-microorganism ratio used to describe biological loading.

Solids retention time: Average time solids remain in a biological system.

Nitrification: Biological oxidation of ammonia to nitrite and nitrate.

Denitrification: Anoxic biological reduction of nitrate to nitrogen gas.

EBPR: Enhanced biological phosphorus removal using phosphorus-accumulating organisms.

Return activated sludge: Settled biological solids returned to the aeration process.

Sidestream: Flow returned to the main plant from sludge or other residuals processing.

Disinfection: Treatment intended to reduce viable pathogens.

Sludge thickening: Reduction of sludge water content before further processing.

Anaerobic digestion: Oxygen-free stabilization of organic solids with biogas production.

Biosolids: Treated sewage solids suitable for regulated beneficial use.

Water reuse: Deliberate use of treated wastewater for a defined beneficial purpose.

Critical control point: A process location at which control is essential to manage risk.

Review Questions

Short Answer Questions

1. Distinguish between hydraulic loading and pollutant mass loading.

2. State the purpose of screening and grit removal.

3. Define solids retention time and explain its importance in nitrification.

4. What conditions are required for effective denitrification?

5. Identify two advantages and two limitations of anaerobic digestion.

Long Answer Questions

1. Explain the functions and design considerations of preliminary, primary, biological and tertiary treatment.

2. Derive or explain the significance of the Monod relationship and discuss its limitations in full-scale design.

3. Compare plug-flow and completely mixed reactor models for biological treatment.

4. Describe an integrated nitrogen- and phosphorus-removal process, including carbon and recycle requirements.

5. Discuss sludge stabilization, dewatering, beneficial use and disposal from a life-cycle perspective.

Analytical / Application Questions

1. A plant treats 12,000 m³/d with an influent BOD concentration of 280 mg/L. Calculate the average BOD load in kg/d and explain why peak flow must also be considered.

2. A proposed reuse scheme combines filtration and UV disinfection. Develop a risk-based verification plan identifying hazards, critical control points, monitoring variables and failure responses.

3. A treatment plant experiences winter ammonia breakthrough and rising sludge in its secondary clarifier. Diagnose possible biological and hydraulic causes and propose an investigation and corrective-action programme.

Multiple Choice Questions with Answers

Question 1

Which parameter is calculated as flow multiplied by concentration, with appropriate unit conversion?

A. Pollutant mass loading

B. Hydraulic retention time

C. Surface overflow rate

D. Solids retention time

Correct Answer: A. Pollutant mass loading

Question 2

What is the principal purpose of grit removal?

A. Oxidation of ammonia

B. Removal of dense inorganic particles that cause abrasion and deposition

C. Disinfection of pathogenic organisms

D. Production of methane

Correct Answer: B. Removal of dense inorganic particles that cause abrasion and deposition

Question 3

In the Monod equation, Ks is the substrate concentration at which:

A. The substrate is completely removed

B. The reactor becomes anaerobic

C. The specific growth rate is approximately half of its maximum value

D. Nitrification ceases permanently

Correct Answer: C. The specific growth rate is approximately half of its maximum value

Question 4

Which condition is essential for denitrification?

A. Anoxic conditions and an electron donor

B. High dissolved oxygen and no carbon

C. Complete absence of nitrate

D. Ultraviolet irradiation

Correct Answer: A. Anoxic conditions and an electron donor

Question 5

A flow of 10,000 m³/d contains 250 mg/L BOD. What is the BOD load?

A. 25 kg/d

B. 250 kg/d

C. 2,500 kg/d

D. 25,000 kg/d

Correct Answer: C. 2,500 kg/d

Question 6

Why is a relatively long solids retention time commonly needed for nitrification?

A. Nitrifiers have comparatively slow growth rates

B. Nitrifiers require large quantities of grit

C. Nitrification occurs only in primary clarifiers

D. Long SRT eliminates the need for oxygen

Correct Answer: A. Nitrifiers have comparatively slow growth rates

Question 7

Which process can produce methane-rich biogas from organic sludge?

A. Gravity thickening

B. Anaerobic digestion

C. UV disinfection

D. Granular filtration

Correct Answer: B. Anaerobic digestion

Question 8

What is a major limitation of chlorination?

A. It cannot reduce pathogens

B. It always increases turbidity

C. It may form disinfection by-products

D. It requires membranes to operate

Correct Answer: C. It may form disinfection by-products

Question 9

For water reuse, treatment requirements should primarily be established from:

A. The colour of the treatment plant buildings

B. The intended end use and associated exposure risks

C. The sludge volume alone

D. The average electricity tariff only

Correct Answer: B. The intended end use and associated exposure risks

Question 10

If a reactor has volume 2,000 m³ and flow 10,000 m³/d, its hydraulic retention time is:

A. 0.2 d

B. 2 d

C. 5 d

D. 20 d

Correct Answer: A. 0.2 d