Twenty Key Wastewater Treatment Indicators Explained: A Must-Know for Wastewater Treatment Experts!

1. BOD (Biochemical Oxygen Demand):

Refers to the concentration of dissolved oxygen needed for microorganisms to biochemically break down organic matter in water under aerobic conditions. To ensure the comparability of BOD measurements, a time period is usually set, during which the dissolved oxygen consumption in water is determined. A common period is five days, referred to as the five-day biochemical oxygen demand (BOD₅). BOD is an environmental monitoring indicator used to evaluate organic pollution in water. Organic substances can be decomposed by microorganisms, consuming oxygen in the process. If the dissolved oxygen in water is insufficient for microbial activity, the water body enters a polluted state.

(1) BOD₅:

A shorthand for “biochemical oxygen demand,” indicating the amount of dissolved oxygen consumed in water at 20°C over 5 days as microorganisms oxidize and decompose organic matter.

First stage: Carbonaceous oxidation (C-BOD).
Second stage: Nitrogenous oxidation (N-BOD).

(2) Significance of BOD:

a. Indicates the amount of organic matter that can be biologically oxidized.
b. Reflects the level of pollution in wastewater and water bodies.
c. Assesses the performance of treatment plants.
d. Used for treatment plant design.
e. Acts as a management parameter.
f. Serves as an emission standard.
g. Functions as a water quality standard indicator.

BOD₅ is frequently used to represent the five-day biochemical oxygen demand. Higher BOD values indicate more organic matter in the water, signifying greater pollution levels.

2. COD (Chemical Oxygen Demand):

Refers to the amount of oxidizing agents consumed to treat a water sample under specified conditions. COD reflects the degree of pollution in water by organic matter. A higher COD value indicates more severe organic pollution. COD is measured in mg/L. Water quality is divided into five categories based on COD values:

Categories I and II: COD ≤ 15 mg/L, suitable for drinking water.
Category III: COD ≤ 20 mg/L.
Category IV: COD ≤ 30 mg/L.
Category V: COD ≤ 40 mg/L (polluted water).

Higher COD values signify more severe pollution.

(1) CODMn / CODCr:

Chemical oxygen demand is denoted as CODMn (using KMnO₄ as the oxidizer) or CODCr (using K₂Cr₂O₇ as the oxidizer). COD is a quick and simple measurement unaffected by water quality limitations and can measure industrial wastewater containing toxic substances, serving as a substitute for BOD.

CODCr approximates the total organic content.
The difference between CODCr and BOD represents organic matter resistant to microbial decomposition.

The BOD₅/COD ratio indicates wastewater’s biodegradability:

BOD₅/COD ≥ 0.3: Suitable for biological treatment.
BOD₅/COD < 0.3: Biological treatment is not recommended.

(2) COD and BOD Relationship:

COD and BOD collectively reflect the total organic matter in water. COD is determined using chemical oxidizers, while BOD is determined through microbial activity. Both values are essential indicators of organic pollution and are widely used for water quality monitoring and environmental assessment.

2. COD (Chemical Oxygen Demand):

Categories I and II: COD ≤ 15 mg/L, suitable for drinking water.
Category III: COD ≤ 20 mg/L.
Category IV: COD ≤ 30 mg/L.
Category V: COD ≤ 40 mg/L (polluted water).
Higher COD values signify more severe pollution.

(1) CODMn / CODCr:

CODCr approximates the total organic content.
The difference between CODCr and BOD represents organic matter resistant to microbial decomposition.

The BOD₅/COD ratio indicates wastewater’s biodegradability:

BOD₅/COD ≥ 0.3: Suitable for biological treatment.
BOD₅/COD < 0.3: Biological treatment is not recommended.

(2) COD and BOD Relationship:

4. TS (Total Solids):

The residue left after evaporating a water sample to dryness at 105-110°C represents the total solids in the water. The difference between total solids and suspended solids represents dissolved substances.

5. VSS (Volatile Suspended Solids):

Volatile suspended solids refer to the amount of organic matter in suspended solids, calculated after heating the sample at 600°C for 30 minutes.

6. Nitrogen (Total Nitrogen, Organic Nitrogen, Ammonia Nitrogen, Nitrite, and Nitrate):

Nitrogen cycles through various forms in nature. Organic nitrogen (e.g., proteins) is hydrolyzed into amino acids and then decomposed into ammonia nitrogen, nitrite, and nitrate through microbial activity. Total nitrogen includes organic nitrogen (protein-based and non-protein nitrogen) and inorganic nitrogen (ammonia, nitrite, and nitrate).

7. Phosphorus (Total Phosphorus, Organic Phosphorus, Inorganic Phosphorus):

Phosphorus is found in detergents, fertilizers, and sewage as phosphates, polyphosphates, and organic phosphorus compounds. Like nitrogen, phosphorus is a vital element for microbial activity but also contributes to water eutrophication.

8. PH Value:

The pH of domestic wastewater is approximately neutral (7). Abnormal pH values in industrial wastewater may affect biological treatment and chemical processes.

9. Alkalinity (as CaCO₃):

Indicates the buffering capacity of wastewater against acidification, mainly contributed by bicarbonates (Ca(HCO₃)₂ and Mg(HCO₃)₂). High alkalinity supports stable biochemical processes.

10. F/M Ratio (Food-to-Microorganism Ratio):

Reflects the organic loading on microorganisms in activated sludge. It is expressed as kgBOD₅/(kgMLVSS·d).

11. VFA (Volatile Fatty Acids):

VFAs are the end products of the fermentation stage in anaerobic processes. They are primarily formed during the acidification phase.

This is a detailed translation of the provided text on water quality parameters, specifically BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), and other related concepts:

1. BOD (Biochemical Oxygen Demand):

(1) BOD₅:

A shorthand for “biochemical oxygen demand,” indicating the amount of dissolved oxygen consumed in water at 20°C over 5 days as microorganisms oxidize and decompose organic matter.

First stage: Carbonaceous oxidation (C-BOD).
Second stage: Nitrogenous oxidation (N-BOD).

(2) Significance of BOD:

BOD₅ is frequently used to represent the five-day biochemical oxygen demand. Higher BOD values indicate more organic matter in the water, signifying greater pollution levels.

2. COD (Chemical Oxygen Demand):

Categories I and II: COD ≤ 15 mg/L, suitable for drinking water.
Category III: COD ≤ 20 mg/L.
Category IV: COD ≤ 30 mg/L.
Category V: COD ≤ 40 mg/L (polluted water).
Higher COD values signify more severe pollution.

(1) CODMn / CODCr:

CODCr approximates the total organic content.
The difference between CODCr and BOD represents organic matter resistant to microbial decomposition.

The BOD₅/COD ratio indicates wastewater’s biodegradability:

BOD₅/COD ≥ 0.3: Suitable for biological treatment.
BOD₅/COD < 0.3: Biological treatment is not recommended.

(2) COD and BOD Relationship:

3. SS (Suspended Solids):

Suspended solids (SS) refer to particles retained by glass fiber filter paper with a pore size of 1μm after passing through a 2mm sieve. Both colloidal and suspended substances are retained by the filter paper.

4. TS (Total Solids):

5. VSS (Volatile Suspended Solids):

Volatile suspended solids refer to the amount of organic matter in suspended solids, calculated after heating the sample at 600°C for 30 minutes.

6. Nitrogen (Total Nitrogen, Organic Nitrogen, Ammonia Nitrogen, Nitrite, and Nitrate):

7. Phosphorus (Total Phosphorus, Organic Phosphorus, Inorganic Phosphorus):

8. pH Value:

The pH of domestic wastewater is approximately neutral (7). Abnormal pH values in industrial wastewater may affect biological treatment and chemical processes.

9. Alkalinity (as CaCO₃):

Indicates the buffering capacity of wastewater against acidification, mainly contributed by bicarbonates (Ca(HCO₃)₂ and Mg(HCO₃)₂). High alkalinity supports stable biochemical processes.

10. F/M Ratio (Food-to-Microorganism Ratio):

Reflects the organic loading on microorganisms in activated sludge. It is expressed as kgBOD₅/(kgMLVSS·d).

11. VFA (Volatile Fatty Acids):

VFAs are the end products of the fermentation stage in anaerobic processes. They are primarily formed during the acidification phase.

12. MLSS and MLVSS:

MLSS (Mixed Liquor Suspended Solids): Represents the concentration of suspended solids in the aeration tank.
MLVSS (Mixed Liquor Volatile Suspended Solids): Indicates the organic fraction of MLSS.

13. SRT (Sludge Retention Time):

The average residence time of microbial cells in the aeration tank.

14. Sludge Volume Index (SVI):

Indicates the settling characteristics of sludge, calculated as:
SVI=SV×10/MLSS\text{SVI} = \text{SV} \times 10 / \text{MLSS}SVI=SV×10/MLSS

15. Sludge Density Index (SDI):

Represents the concentration of sludge in settled volume after 30 minutes.
SDI=100/SVI\text{SDI} = 100 / \text{SVI}SDI=100/SVI

16. Sludge Load (Ns):

Describes the organic load processed per unit mass of activated sludge in the aeration tank.

17. Volume Load (Fr):

The organic load per cubic meter of reactor volume, expressed as kgBOD₅/(m³·d).

18. Sludge Load (Ns):

The sludge load refers to the amount of five-day biochemical oxygen demand (BOD₅) handled per kilogram of activated sludge in the aeration tank per unit of time. Its unit is typically expressed as kg/(kg·d). Sludge load (Ns) represents the amount of pollutants removed by a unit mass of activated sludge within a given time. In the context of microbial metabolism, it corresponds to the F/M ratio, expressed as kgCOD(BOD)/(kg sludge·d). Sludge load varies across different stages of sludge growth, and its effectiveness in purification also differs. Thus, it is a critical parameter for designing and operating the activated sludge process.

In general, when the sludge load ranges between 0.3 and 0.5 kg/(kg·d), the BOD₅ removal rate can exceed 90%, with a Sludge Volume Index (SVI) between 80 and 150, indicating good adsorption and settling performance of the sludge.

Calculation Formula:

Ns=F/M=QSVXNs = F/M = \frac{QS}{VX}Ns=F/M=VXQS Where:

Ns: Sludge load, kgCOD(BOD)/(kg sludge·d).
Q: Daily influent volume, m³/d.
S: COD (BOD) concentration, mg/L.
V: Effective aeration tank volume, m³.
X: Sludge concentration, mg/L.

19. Volume Load (Fr):

Volume load refers to the amount of organic matter processed per cubic meter of tank volume per day. It usually indicates the daily five-day biochemical oxygen demand (BOD₅) load (for aeration tanks, biological contact oxidation tanks, and biofilters) or volatile suspended solids load (for sludge digesters). Its unit is typically expressed as kg/(m³·d). Volume load represents the weight of pollutants that can be removed per cubic meter of reactor volume within a given time.

Formula:

Fr=Ns×NwFr = Ns \times NwFr=Ns×Nw
Where:

Fr: Volume load, kgBOD₅/(m³·d) or kgCOD/(m³·d).
Ns: Sludge load, kgBOD₅/(kgMLSS·d).
Nw: Mixed liquor suspended solids (MLSS) concentration, g/L or kg/m³.

Simplified calculation:
Fr=(q1−q2)×241000VFr = \frac{(q_1 – q_2) \times 24}{1000V}Fr=1000V(q1−q2)×24
Where:

q₁: Influent concentration, mg/L.
q₂: Effluent concentration, mg/L.
V: Aeration tank volume, m³.

Volume load provides a straightforward and intuitive way to evaluate the actual treatment load of biochemical facilities and the operational management quality under identical conditions. In coking systems, COD volume load is often used as a comprehensive evaluation parameter due to its ease of measurement.

20. Organic Load (F/M):

Organic load refers to the amount of organic matter removed per unit volume of filter media (or tank) per unit of time. It is an essential parameter in designing and operating biological filters (or aeration tanks).

There are two methods to express organic load:

Based on the amount of organic matter entering the filter.
Based on the amount of organic matter removed by the filter.

For the first method, the removal efficiency must be specified. The second method essentially reflects the oxidation capacity. Organic matter is typically expressed in terms of BOD₅ or COD, so it is also referred to as the BOD or COD load. Its unit is expressed as g (or kg)/(m³ filter media·d) for filters, or g (or kg)/(m³ tank volume·d) for aeration tanks. When calculating the organic matter load, organic matter in the recirculated flow is generally excluded (if a recirculation system is used).