Environmental Chemistry, Microbiology and Process Fundamentals

Introduction and Context

Environmental engineering is fundamentally concerned with the movement, transformation, and control of chemical and biological substances in water, wastewater, soil, air, and engineered treatment systems. Successful design requires more than knowledge of individual treatment units. It requires an understanding of equilibrium, reaction kinetics, thermodynamics, acid–base chemistry, oxidation–reduction reactions, adsorption, microbial ecology, mass balances, reactor behaviour, and contaminant fate.

These subjects are interconnected. For example, the removal of ammonia from wastewater depends on acid–base speciation, oxygen availability, microbial growth kinetics, redox conditions, temperature, and reactor configuration. Similarly, the fate of a metal in a river may be controlled by pH, oxidation state, precipitation, adsorption to suspended solids, and biological uptake.

This chapter develops the process fundamentals needed to analyse such systems quantitatively and critically.

Learning Objectives

By the end of this chapter, the reader should be able to:

1. Explain chemical equilibrium, thermodynamic feasibility, and reaction kinetics in environmental systems.

2. Apply acid–base, redox, precipitation, and adsorption concepts to environmental problems.

3. Describe microbial metabolism, ecological interactions, and biodegradation pathways.

4. Formulate and interpret mass balances for environmental reactors.

5. Distinguish batch, completely mixed, and plug-flow reactor behaviour.

6. Use first-order contaminant-fate models while recognising their assumptions and limitations.

7. Integrate chemistry, microbiology, and process theory in treatment-system analysis.

1. Chemical Equilibrium and Thermodynamic Foundations

1.1 Equilibrium as a limiting condition

A chemical reaction reaches equilibrium when its forward and reverse reaction rates are equal. Equilibrium does not mean that reactions have stopped; rather, dynamic molecular activity continues while macroscopic concentrations remain constant.

For the general reaction

\[

aA+bB \rightleftharpoons cC+dD

\]

the thermodynamic equilibrium constant is

\[

K=\frac{a_C^c a_D^d}{a_A^a a_B^b}

\]

where \(a_i\) is the activity of species \(i\), a dimensionless measure of effective concentration. In dilute solutions, activity is often approximated by concentration divided by a standard concentration. At higher ionic strength, activity coefficients should be used.

The reaction quotient \(Q\) has the same form as \(K\), but uses current activities. If \(Q<K\), the reaction tends to proceed forward; if \(Q>K\), it tends to proceed in the reverse direction; and if \(Q=K\), the system is at equilibrium.

Equilibrium calculations are applicable only when the system is sufficiently well mixed, temperature is specified, relevant species are included, and the reaction is not being substantially altered by continuous removal or addition of reactants.

1.2 Thermodynamics and environmental feasibility

The Gibbs free-energy change is

\[

\Delta G=\Delta G^\circ+RT\ln Q

\]

where \(\Delta G\) is the actual free-energy change, \(\Delta G^\circ\) is the standard free-energy change, \(R\) is the gas constant, \(T\) is absolute temperature, and \(Q\) is the reaction quotient. A negative value indicates thermodynamic favourability in the specified direction, but it does not indicate how rapidly the reaction will occur.

At equilibrium, \(\Delta G=0\), and therefore

\[

\Delta G^\circ=-RT\ln K.

\]

This distinction between thermodynamic feasibility and kinetic rate is central to environmental engineering. Oxidation of organic matter by oxygen may be energetically favourable, yet treatment performance may be poor if oxygen transfer is inadequate or if microbial populations have not developed.

1.3 Temperature dependence

The van’t Hoff relationship describes the approximate effect of temperature on an equilibrium constant:

\[

\frac{d\ln K}{dT}=\frac{\Delta H^\circ}{RT^2}

\]

where \(\Delta H^\circ\) is the standard enthalpy change. Rate constants are commonly adjusted using the empirical relationship

\[

k_T=k_{T_0}\theta^{(T-T_0)}

\]

where \(\theta\) is a dimensionless temperature coefficient, commonly near 1.03–1.10 for biological processes, although its value must be established for the specific process. Temperature corrections should not be extrapolated beyond validated ranges.

2. Reaction Kinetics

2.1 Rate laws

Kinetics describes the speed of transformation. For a reactant concentration \(C\), a general rate law is

\[

-r_A=kC_A^n

\]

where \(r_A\) is the disappearance rate, \(k\) is the rate constant, and \(n\) is the reaction order.

For a first-order reaction,

\[

-r=kC.

\]

In a batch system with constant volume,

\[

\frac{dC}{dt}=-kC,

\qquad C=C_0e^{-kt}.

\]

The half-life is

\[

t_{1/2}=\frac{\ln 2}{k}.

\]

The units of \(k\) depend on reaction order: time⁻¹ for first order and concentration⁻¹ time⁻¹ for second order. A first-order approximation is often used when one reactant is present in large excess; this is called pseudo-first-order kinetics.

2.2 Monod kinetics and microbial growth

Microbial growth on a limiting substrate is commonly represented by 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 substrate concentration, and \(K_S\) is the half-saturation constant. When \(S=K_S\), \(\mu=0.5\mu_{\max}\).

A more complete expression may include decay and environmental limitations:

\[

\mu_{\text{net}}=\mu-b

\]

where \(b\) is the endogenous decay coefficient. In real systems, growth may also be limited by oxygen, nutrients, pH, temperature, toxicity, or electron acceptor availability.

Kinetic parameters are not universal constants. They depend on microbial community, wastewater composition, temperature, measurement method, and reactor history.

3. Acid–Base Chemistry and pH

3.1 Definitions and water dissociation

The pH is defined as

\[

pH=-\log_{10}a_{H^+}.

\]

For water at 25°C,

\[

K_w=a_{H^+}a_{OH^-}\approx 1.0\times10^{-14}.

\]

For dilute neutral water, \([H^+]=[OH^-]=10^{-7}\) mol/L and pH is approximately 7. The numerical value of neutral pH changes with temperature because \(K_w\) changes.

For a weak acid,

\[

HA\rightleftharpoons H^++A^-

\]

\[

K_a=\frac{a_{H^+}a_{A^-}}{a_{HA}},

\qquad pK_a=-\log K_a.

\]

The Henderson–Hasselbalch equation is

\[

pH=pK_a+\log_{10}\left(\frac{[A^-]}{[HA]}\right).

\]

It is most reliable when both acid and conjugate-base forms are present in appreciable amounts and activity effects are modest.

3.2 Alkalinity and buffering

Alkalinity is the acid-neutralising capacity of water, usually expressed as mg/L as CaCO₃. It is associated primarily with bicarbonate, carbonate, hydroxide, phosphate, ammonia, and organic functional groups.

Alkalinity is not the same as pH. A solution may have a near-neutral pH but very low buffering capacity, making its pH vulnerable to acid addition. Wastewater treatment processes such as nitrification consume alkalinity because ammonia oxidation produces acidity. Sufficient alkalinity must therefore be maintained to prevent harmful pH decline.

3.3 Environmental significance of speciation

Speciation determines toxicity, volatility, solubility, mobility, and biodegradability. The un-ionised fraction of ammonia, for example, is generally more toxic to aquatic organisms than ammonium ion. The relative abundance of these forms depends on pH and temperature. Metal toxicity likewise depends on free-ion concentration rather than simply total metal concentration.

4. Oxidation–Reduction Chemistry

Redox reactions involve electron transfer. Oxidation is loss of electrons, while reduction is gain of electrons. Environmental redox processes are often written as coupled half-reactions, such as:

\[

\text{Oxidant}+ne^-\rightarrow\text{Reduced product}.

\]

Microorganisms obtain energy by coupling oxidation of an electron donor, often organic carbon or hydrogen, with reduction of an electron acceptor such as oxygen, nitrate, manganese(IV), iron(III), sulfate, or carbon dioxide.

A useful conceptual sequence in oxygen-depleted sediments is:

1. oxygen reduction;

2. nitrate reduction and denitrification;

3. manganese reduction;

4. iron reduction;

5. sulfate reduction;

6. methanogenesis.

The sequence is controlled by energetic yield, availability of electron acceptors, transport, and microbial competition. Redox potential, commonly denoted Eh, provides an indication of oxidising or reducing conditions, but interpretation requires attention to pH, equilibration, electrode limitations, and the specific redox couple.

Redox chemistry is important in arsenic and chromium mobility, nitrogen removal, iron and manganese precipitation, sulfide formation, and the transformation of organic contaminants.

5. Precipitation, Dissolution and Adsorption

5.1 Solubility equilibria

For a sparingly soluble solid,

\[

M_xA_y(s)\rightleftharpoons xM^{z+}+yA^{w-}

\]

its solubility product is

\[

K_{sp}=a_{M^{z+}}^x a_{A^{w-}}^y.

\]

The ion activity product, or IAP, is compared with \(K_{sp}\). If IAP exceeds \(K_{sp}\), precipitation is thermodynamically possible; if it is below \(K_{sp}\), dissolution is favoured. Supersaturation alone does not guarantee rapid precipitation because nucleation and crystal growth may be slow.

5.2 Adsorption

Adsorption is the accumulation of a substance at an interface, such as the surface of activated carbon, clay, iron hydroxide, or biological solids. It differs from absorption, in which a substance enters the bulk phase.

A linear isotherm is

\[

q=K_dC_e

\]

where \(q\) is mass adsorbed per mass of solid, \(C_e\) is equilibrium dissolved concentration, and \(K_d\) is a distribution coefficient.

The Freundlich isotherm is

\[

q=K_FC_e^{1/n},

\]

while the Langmuir model is

\[

q=\frac{q_{\max}bC_e}{1+bC_e}.

\]

The Langmuir model assumes a finite number of equivalent sites and monolayer coverage; these assumptions may not hold for heterogeneous natural solids. Adsorption capacity is influenced by pH, ionic strength, competing solutes, dissolved organic matter, particle size, and contact time.

6. Environmental Microbiology and Biodegradation

6.1 Microbial groups and metabolism

Environmental microorganisms include bacteria, archaea, fungi, algae, protozoa, and viruses. Bacteria and archaea are especially important in wastewater treatment, while fungi can contribute to decomposition of complex organic compounds. Protozoa graze on bacteria and improve effluent clarification.

Microbial metabolism can be described using four functional components:

• electron donor: the substance oxidised for energy;
• electron acceptor: the substance reduced;
• carbon source: inorganic carbon or organic carbon used for cell synthesis;
• energy-conservation mechanism: generally respiration or fermentation.

Aerobic heterotrophs oxidise organic matter using oxygen. Nitrifying organisms oxidise ammonia to nitrite and nitrate. Denitrifying organisms reduce nitrate to nitrogen gas under anoxic conditions using biodegradable organic carbon. Methanogens produce methane under strongly reducing, oxygen-free conditions.

6.2 Biofilms and microbial ecology

Biofilms are structured microbial communities embedded in extracellular polymeric substances. They contain gradients of oxygen, substrates, pH, and redox potential. These gradients permit aerobic, anoxic, and anaerobic reactions to occur within millimetres of one another.

Microbial ecology involves competition, syntrophy, predation, inhibition, and horizontal gene transfer. In anaerobic digestion, for example, fermenters produce volatile fatty acids, acetogens convert intermediates to acetate and hydrogen, and methanogens produce methane. These groups depend on one another; accumulation of intermediates can destabilise the process.

6.3 Biodegradation limitations

Biodegradation is controlled by contaminant bioavailability, microbial acclimation, toxicity, electron acceptors, nutrients, temperature, pH, and mass transfer. A contaminant may be chemically degradable but persist because it is strongly sorbed or inaccessible within an aged soil matrix. Conversely, transformation may produce metabolites that are more mobile or toxic than the parent compound.

7. Mass Balances and Reactor Fundamentals

7.1 General mass balance

The conservation equation for a control volume is

\[

\text{Accumulation}= ext{inflow}-\text{outflow}+\text{generation}-\text{consumption}.

\]

For a completely mixed tank of volume \(V\), flow \(Q\), influent concentration \(C_i\), effluent concentration \(C\), and reaction rate \(r(C)\),

\[

V\frac{dC}{dt}=QC_i-QC+Vr(C).

\]

If \(r(C)=-kC\) for first-order decay, then

\[

V\frac{dC}{dt}=Q(C_i-C)-VkC.

\]

At steady state,

\[

C=\frac{C_i}{1+k\tau},

\qquad \tau=\frac{V}{Q}.

\]

Here \(\tau\) is the hydraulic retention time. This expression assumes constant volume, steady flow, complete mixing, first-order kinetics, and no other sources or sinks.

7.2 Ideal reactor models

In a batch reactor, there is no flow during the reaction period. Concentration changes with time and mixing is usually assumed to be uniform.

In a completely mixed flow reactor (CMFR), influent is instantaneously mixed throughout the tank, so effluent concentration equals tank concentration. It is useful for systems requiring stable conditions and for modelling activated sludge tanks.

In an ideal plug-flow reactor (PFR), fluid elements move through the reactor with negligible axial mixing. For first-order decay and constant velocity,

\[

C=C_i e^{-k\tau}.

\]

For equal volume and first-order removal, a PFR generally provides greater removal than one CMFR because the concentration remains higher near the inlet, producing a higher average reaction rate. Multiple CMFRs in series approach plug-flow behaviour.

7.3 Worked example: first-order treatment

A wastewater contains a biodegradable contaminant at 100 mg/L. It enters a completely mixed reactor with a hydraulic retention time of 4 h. The contaminant follows first-order decay with \(k=0.20\) h⁻¹. Assuming steady state and no additional loading,

\[

C=\frac{100}{1+(0.20)(4)}

=\frac{100}{1.8}=55.6\text{ mg/L}.

\]

For an ideal plug-flow reactor with the same retention time,

\[

C=100e^{-(0.20)(4)}=100e^{-0.8}=44.9\text{ mg/L}.

\]

The difference arises from the concentration profile, not from a different kinetic constant. Real reactors deviate from ideal behaviour because of short-circuiting, dead zones, recycle, dispersion, and variable loading.

8. Contaminant Fate and Transport

A contaminant’s environmental fate is governed by advection, dispersion, sorption, reaction, volatilisation, dilution, and phase transfer. A simplified one-dimensional advection–dispersion–reaction equation is

\[

\frac{\partial C}{\partial t}=D\frac{\partial^2C}{\partial x^2}-v\frac{\partial C}{\partial x}-kC,

\]

where \(C\) is concentration, \(D\) is dispersion coefficient, \(v\) is average pore-water velocity, and \(k\) is a first-order decay coefficient. The equation assumes homogeneous properties, linear processes, constant coefficients, and a single dissolved contaminant undergoing first-order reaction.

Sorption can retard movement. Under linear equilibrium sorption, the retardation factor is

\[

R=1+\frac{\rho_bK_d}{\theta}

\]

where \(\rho_b\) is dry bulk density, \(K_d\) is the distribution coefficient, and \(\theta\) is volumetric water content. The expression is dimensionless when units are consistent. Retardation slows contaminant migration relative to groundwater, but it does not necessarily destroy mass. Desorption may later produce a persistent contaminant source.

Risk assessment must distinguish concentration, exposure, hazard, and risk. A chemical can be hazardous but pose low risk if exposure is negligible; conversely, a moderately hazardous chemical can create substantial risk through persistent, widespread exposure.

9. Integrated Applied Case: Nitrification Failure in a Wastewater Plant

Background

This illustrative case concerns a municipal activated-sludge plant receiving a sudden increase in ammonia loading. The plant historically achieved nitrification, but effluent ammonia began to rise.

Problem

Operators initially increased aeration. Dissolved oxygen improved, but ammonia removal remained poor. Investigation showed that alkalinity had declined and reactor pH had fallen from 7.4 to 6.3.

Analysis

Nitrification is an aerobic biological oxidation process. It consumes oxygen and alkalinity and is sensitive to pH, temperature, toxic compounds, sludge age, and ammonia loading. At low pH, nitrifier activity decreases, while the available carbonate buffering system becomes less effective. The increased ammonia load also raised the oxygen demand and required a larger nitrifying population.

A simplified stoichiometric representation is:

\[

NH_4^+ + 2O_2 \rightarrow NO_3^- + 2H^+ + H_2O.

\]

The production of hydrogen ions explains alkalinity consumption. If the influent contains insufficient alkalinity, nitrification can cause a self-reinforcing decline in pH and activity.

Approach and solution

The plant implemented four measures:

1. verified ammonia, alkalinity, pH, temperature, and dissolved oxygen measurements;

2. added alkalinity using a controlled chemical feed;

3. increased solids retention time to retain slow-growing nitrifiers;

4. reviewed aeration capacity and screened for inhibitory industrial discharges.

The operators did not rely solely on a single online sensor. Laboratory checks were used to validate instrument readings, and trends were evaluated rather than reacting to isolated values.

Outcome

As alkalinity and pH recovered, nitrifier activity improved. Ammonia removal gradually returned because biological recovery required time; immediate chemical correction could not instantly rebuild the microbial population.

Learning points

The case demonstrates that treatment failure is often multi-causal. Oxygen addition alone cannot solve a problem caused by pH, alkalinity, microbial washout, toxicity, or insufficient solids retention time. Effective diagnosis combines chemistry, microbiology, hydraulics, instrumentation, and operational history.

10. Limitations, Governance and Contemporary Practice

Environmental models are simplifications. Equilibrium models may fail where reactions are kinetically limited. First-order models may conceal substrate limitation, inhibition, microbial adaptation, or multiple contaminant pools. Ideal reactor models do not represent hydraulic anomalies. Laboratory-derived parameters may not transfer directly to full-scale facilities.

Professional practice therefore requires calibration, sensitivity analysis, uncertainty reporting, and independent verification. Sampling plans should account for temporal variability, preservation requirements, detection limits, and representative locations. Data quality is a technical and governance issue because treatment decisions affect public health, ecosystems, and regulatory compliance.

Regulatory limits should be interpreted with attention to whether they apply to total concentration, dissolved concentration, bioavailable concentration, instantaneous discharge, or long-term average. Chemical substitution and emerging contaminants also raise ethical questions: replacing a regulated substance with an insufficiently studied alternative may transfer rather than reduce risk.

Contemporary practice increasingly uses online sensors, respirometry, molecular microbial tools, high-resolution mass spectrometry, digital twins, and machine-learning models. Artificial intelligence can assist anomaly detection, soft sensing, and predictive control, but it should not replace mass-balance checks, process understanding, or professional accountability. Models trained on historical data may fail during unusual storms, industrial discharges, climate-driven temperature changes, or changes in influent composition.

Best practice is to combine mechanistic models with validated data, maintain transparent assumptions, document uncertainty, and use adaptive monitoring. In all applications, the conservation laws and chemical plausibility of a result should be checked before operational action is taken.

Chapter Summary

Environmental engineering processes are governed by linked chemical, biological, and physical mechanisms. Equilibrium determines the thermodynamically preferred state, whereas kinetics determines the rate at which that state is approached. Gibbs free energy helps assess feasibility but cannot predict treatment speed. Acid–base chemistry controls pH, buffering, speciation, toxicity, and process stability. Redox reactions determine the sequence of electron-acceptor use and influence nutrient, metal, sulfur, and carbon transformations.

Precipitation and adsorption control the partitioning of contaminants between dissolved and solid phases. Microorganisms transform pollutants through respiration, fermentation, synthesis, and decay; their performance depends on substrate, electron acceptors, environmental conditions, community structure, and mass transfer. Mass balances provide the foundation for reactor analysis, while batch, CMFR, and PFR models describe different hydraulic behaviours. First-order and Monod equations are useful approximations, but their assumptions must be checked.

Contaminant-fate models integrate advection, dispersion, sorption, and reaction. Professional application requires attention to uncertainty, data quality, regulation, ethics, and the limitations of idealised models. The most reliable environmental decisions result from integrating chemistry, microbiology, hydraulics, monitoring, and engineering judgement.

Key Terms

Equilibrium: Dynamic condition in which forward and reverse reaction rates are equal.

Activity: Effective thermodynamic concentration of a chemical species.

Reaction quotient: Ratio of current product and reactant activities.

Gibbs free energy: Thermodynamic quantity indicating reaction feasibility under specified conditions.

Kinetics: Study of reaction rates and mechanisms.

First-order reaction: Reaction whose rate is proportional to one reactant concentration.

Monod equation: Saturation-based expression relating microbial growth rate to substrate concentration.

pH: Negative logarithm of hydrogen-ion activity.

Alkalinity: Acid-neutralising capacity of water.

Speciation: Distribution of an element among its chemical forms.

Redox reaction: Reaction involving transfer of electrons.

Electron acceptor: Species reduced during energy-yielding metabolism.

Solubility product: Equilibrium constant for dissolution of a sparingly soluble solid.

Adsorption: Accumulation of a substance at a surface.

Biofilm: Structured microbial community attached to a surface and embedded in extracellular material.

Hydraulic retention time: Reactor volume divided by flow rate.

Completely mixed flow reactor: Reactor in which concentration is uniform throughout the tank.

Plug-flow reactor: Ideal reactor with negligible axial mixing.

Retardation factor: Ratio describing the slowing of sorbed-contaminant movement relative to water.

Biodegradation: Biological transformation or destruction of a compound.

Review Questions

Short Answer Questions

1. Distinguish thermodynamic feasibility from reaction kinetics.

2. Define alkalinity and explain why it is important during nitrification.

3. What is the difference between adsorption and absorption?

4. State the assumptions of a completely mixed flow reactor model.

5. Why can a contaminant remain persistent even when biodegradation is thermodynamically favourable?

Long Answer Questions

1. Explain how pH affects chemical speciation, toxicity, adsorption, and biological treatment.

2. Compare batch, completely mixed, and plug-flow reactors, including their concentration profiles and practical uses.

3. Describe the roles of electron donors and acceptors in aerobic, anoxic, and anaerobic microbial processes.

4. Explain how precipitation and adsorption influence the mobility of metals in natural waters and soils.

5. Discuss the strengths and limitations of first-order and Monod kinetic models in environmental engineering.

Analytical / Application Questions

1. A first-order contaminant enters a CMFR at 80 mg/L. If the hydraulic retention time is 3 h and the rate constant is 0.25 h⁻¹, calculate the steady-state effluent concentration. State your assumptions.

2. A groundwater contaminant has \(K_d=0.40\) L/kg, dry bulk density 1.6 kg/L, and volumetric water content 0.30. Calculate the retardation factor and interpret its meaning.

3. A nitrifying reactor experiences falling pH despite adequate dissolved oxygen. Develop a diagnostic plan that distinguishes alkalinity deficiency, insufficient sludge age, toxicity, temperature effects, and sensor error.

Multiple Choice Questions with Answers

Question 1

At equilibrium for a reversible reaction, which condition applies?

A. The forward reaction has stopped

B. The reverse reaction has stopped

C. Forward and reverse reaction rates are equal

D. All reactants have been consumed

Correct Answer: C. Forward and reverse reaction rates are equal

Question 2

For a first-order batch reaction, the half-life is

A. k/ln 2

B. ln 2/k

C. 2k

D. 1/(2k)

Correct Answer: B. ln 2/k

Question 3

When pH equals pKa for a weak acid system, the ratio [A−]/[HA] is

A. 0.01

B. 0.1

C. 1

D. 10

Correct Answer: C. 1

Question 4

Which process generally occurs under anoxic conditions?

A. Nitrification using oxygen

B. Denitrification using nitrate as electron acceptor

C. Methane combustion

D. Oxygenic photosynthesis only

Correct Answer: B. Denitrification using nitrate as electron acceptor

Question 5

For a first-order contaminant in a CMFR, the steady-state concentration is

A. Ci(1+kτ)

B. Ci/(1−kτ)

C. Ci/(1+kτ)

D. Ci e^(kτ)

Correct Answer: C. Ci/(1+kτ)

Question 6

In the Langmuir adsorption model, qmax represents

A. The maximum reaction rate

B. The maximum adsorption capacity

C. The equilibrium pH

D. The influent concentration

Correct Answer: B. The maximum adsorption capacity

Question 7

If the ion activity product exceeds the solubility product, the solution is

A. Undersaturated

B. At neutral pH

C. Supersaturated with respect to the solid

D. Guaranteed to precipitate immediately

Correct Answer: C. Supersaturated with respect to the solid

Question 8

In the Monod equation, when S equals KS, the specific growth rate is

A. Zero

B. One-quarter of μmax

C. One-half of μmax

D. Equal to μmax squared

Correct Answer: C. One-half of μmax

Question 9

A retardation factor greater than one indicates that the contaminant

A. Moves faster than groundwater

B. Is not affected by sorption

C. Moves more slowly than groundwater because of sorption

D. Cannot biodegrade

Correct Answer: C. Moves more slowly than groundwater because of sorption

Question 10

Which statement best describes alkalinity?

A. It is identical to pH

B. It is the acid-neutralising capacity of water

C. It is the oxygen concentration of water

D. It is the total concentration of microorganisms

Correct Answer: B. It is the acid-neutralising capacity of water