20 key control parameters for wastewater treatment

I. BOD5: Biochemical Oxygen Demand (BOD), representing the amount of dissolved oxygen consumed by microorganisms in water at 20℃ over 5 days to oxidize and decompose organic matter. The first stage is carbonization (C-BOD), and the second stage is digestion (N-BOD).

Significance of BOD:

1. The amount of organic matter that can be biologically oxidized and decomposed;

2. Reflects the pollution level of wastewater and water bodies;

3. Determines the effectiveness of treatment plants;

4. Used in treatment plant design;

5. Wastewater treatment management indicator;

6. Emission standard indicator;

7. Water quality standard indicator for water bodies.

II. CODMn/CODCr: Chemical Oxygen Demand (COD), where oxidants are KMnO4 and K2Cr2O7. COD determination is simple and rapid, not subject to water quality limitations, and can measure industrial wastewater containing biologically toxic substances. It can also be considered as the amount of reductant.

CODCr can be approximately considered as the total amount of organic matter. The difference between CODCr and BOD represents the organic matter in wastewater that is difficult to decompose by microorganisms. The BOD/CODCr ratio represents the biodegradability of wastewater. When BOD/CODCr ≥ 0.3, the wastewater is considered to have good biodegradability; when BOD/CODCr < 0.3, the wastewater is considered to have poor biodegradability and is not suitable for biological treatment.

III. SS: Suspended Solids, the substance in water that passes through a 2mm sieve and is retained by a glass fiber filter paper with a pore size of 1μm. Colloidal substances are present in both the filtrate (dissolved substances) and the retained suspended solids, but most consider colloidal substances to be retained by the filter paper like suspended solids.

IV. TS: Total Solids, the residue remaining after evaporation and drying of a water sample. It is the total amount of solid matter remaining after evaporating the water sample to dryness at 105-110℃. The amount of dissolved solids is equal to the total solids minus the suspended solids.

V. Volatile Total Solids (VTS)/Volatile Suspended Solids (VSS): The substance volatilized at 600℃±25℃ for 30min from total solids or suspended solids, representing the amount of organic matter (VTS for the former, VSS for the latter). The difference between the total solids before and after incineration is called the ash residue, representing the inorganic part.

VI. Total Nitrogen, Organic Nitrogen, Ammonia Nitrogen, Nitrite Nitrogen, Nitrate Nitrogen: Nitrogen undergoes cyclical transformation in various forms in nature.

Organic nitrogen, such as protein, hydrolyzes into amino acids, which are decomposed into ammonia nitrogen under the action of microorganisms. Ammonia nitrogen is converted into nitrite nitrogen (NO2-) and nitrate nitrogen (NO3-) under the action of nitrifying bacteria. In addition, NO2- and NO3- are converted into N2 under anaerobic conditions under the action of denitrifying bacteria.

Total nitrogen = organic nitrogen (organic nitrogen = protein nitrogen + non-protein nitrogen) + inorganic nitrogen (inorganic nitrogen = ammonia nitrogen + NO2- + NO3-). Nitrogen is an essential element for bacterial reproduction. When the nitrogen content in industrial wastewater is insufficient, nitrogen needs to be supplemented artificially during biological treatment. Conversely, nitrogen is also one of the elements that cause eutrophication pollution of water bodies.

VII. Total Phosphorus, Organic Phosphorus, Inorganic Phosphorus: Feces, detergents, and fertilizers contain a large amount of phosphorus. Wastewater contains inorganic phosphate salts such as phosphate and polyphosphate, and organic phosphorylated compounds such as phospholipids. Like nitrogen, phosphorus is an essential element for wastewater biological treatment, and it is also one of the elements that cause eutrophication pollution of enclosed water bodies.

VIII. pH: The pH of domestic wastewater is around 7. The discharge of strongly acidic or alkaline industrial wastewater causes changes in pH. Abnormal pH values or large pH changes will affect biological treatment. In addition, pH is an important operating condition in physicochemical treatment.

IX. Alkalinity (CaCO3): Represents the ability of wastewater to neutralize acids, usually expressed as CaCO3 content. Wastewater mainly contains Ca(HCO3)2 and Mg(HCO3)2 alkalinity. Higher alkalinity means stronger buffering capacity, which can meet the alkalinity consumption of the wastewater nitrification reaction. In sludge digestion, it buffers the acidification caused by overload operation, which is beneficial to the stability of the digestion process.

X. F/M: Organic Loading Rate (F/M), also known as sludge loading. F refers to organic matter, and M refers to microorganisms. Organic loading rate F/M: The amount of organic matter borne by a unit weight of activated sludge per unit time, or the amount of organic matter removed per unit effective volume of the bioreactor per unit time, unit kgBOD5/(kgMLVSS·d). "F" refers to "amount of organic matter", and "M" refers to "amount of microorganisms". The ratio of the two is used to reflect the sludge load. The main thing to grasp in biological treatment is the concept of sludge age and BOD organic load, everything is related to this.

XI. VFA: Volatile Fatty Acid (VFA) is the end product of the fermentation stage in anaerobic biological treatment. In the fermentation stage, the small molecular compounds produced in the hydrolysis stage are converted into simpler end products, mainly volatile fatty acids, inside the cells of the fermenting bacteria and secreted outside the cells, which is the acidification stage. In the initial stage of reactor startup, the pH of the influent must be controlled, mainly by adding sodium hydroxide to control the pH of the influent, so that the reaction can be maintained in a relatively stable environment.

12. MLSS: MLSS stands for mixed liquor suspended solids, also known as mixed liquor sludge concentration. It represents the total weight (mg/L) of activated sludge solids contained in a unit volume of mixed liquor in the aeration tank. Due to its relatively simple and easy measurement method, this indicator is widely used. The mixed liquor suspended solids concentration (MLSS) is an important design and operating parameter for activated sludge treatment systems. For domestic sewage, MLVSS/MLSS = 0.7, where MLVSS refers to mixed liquor volatile suspended solids.

13. MLVSS: This is the mixed liquor volatile suspended solids concentration. It represents the concentration of organic solid matter in the mixed liquor activated sludge. Compared to MLSS, this indicator provides a more accurate representation of the quantity of the active portion of the activated sludge.

14. Sludge settling ratio (SV): This refers to the ratio of the volume of settled sludge to the volume of the mixed liquor sample after allowing a well-mixed sample of activated sludge from the aeration tank to settle in a 1000mL graduated cylinder for 30 minutes. It is also known as sludge settling volume (SV30), expressed as mL/L.

Because sludge settles after 30 minutes, it generally reaches or approaches its maximum density. Therefore, this time is generally used as the standard time for measuring this indicator. It can also be measured at 15 minutes. The sludge settling ratio SV30 is a very important indicator. By observing the settling ratio, many problems with the sludge properties can be identified, such as whether the supernatant is clear, whether it contains difficult-to-settle suspended flocs, the size and compactness of the flocs, etc. The sludge settling ratio roughly reflects the amount of sludge in the reactor and can be used to control sludge discharge changes and promptly reflect abnormal situations such as sludge bulking.

15. Sludge volume index (SVI): Common measurement methods: a. Take a mixed liquor sample from the outlet of the aeration tank; b. Measure MLSS; c. Measure the SV% of the sample and read the volume (mL) of the settled sludge. Calculation formula: Volume of settled sludge (mL) / MLSS (mL); The SVI value is an important parameter for judging the sludge settling and concentration performance. It is generally considered that when the SVI value is 100-150, the sludge settling performance is good; when the SVI value > 200, the sludge settling performance is poor; when the SVI value is too low, the sludge flocs are small, compact, and contain more inorganic matter, resulting in poor sludge activity.

16. Sludge density index (SDI): The number of grams of activated sludge suspended solids contained in 100 mL of settled sludge after the mixed liquor in the aeration tank has settled for 30 minutes is called the sludge density index (SDI). Numerically, SDI is equal to the reciprocal of the sludge volume index (SVI) multiplied by 100. That is: SDI = 100/SVI.

17. Sludge load (Ns): The number of kilograms of five-day biochemical oxygen demand (BOD5) borne per kilogram of activated sludge per unit time in the aeration tank. The unit of measurement is usually kg/(kg·d). Sludge load (Ns) refers to the amount of pollutants removed per unit mass of activated sludge per unit time.

The meaning of sludge load in terms of microbial metabolism is the F/M ratio, unit kgCOD(BOD)/(kg sludge·d); In different stages of sludge growth, the sludge load is different, and the purification effect is also different. Therefore, sludge load is one of the main parameters for the design and operation of the activated sludge process. Generally speaking, when the sludge load is in the range of 0.3-0.5 kg/(kg·d), the BOD5 removal rate can reach more than 90%, SVI is 80-150, and the adsorption and sedimentation performance of the sludge is good.

Calculation method of sludge load: Ns = F/M = QS/(VX) where: Ns is the sludge load, kgCOD(BOD)/(kg sludge·d); Q is the daily influent volume, m3/d; S is the COD(BOD) concentration, mg/L; V is the effective volume of the aeration tank, m3; X is the sludge concentration, mg/L.

18. Volumetric load (Fr): The amount of organic matter borne per cubic meter of tank volume per day, generally referring to the number of kilograms of five-day biochemical oxygen demand (BOD5) borne per unit time (aeration tank, biological contact oxidation tank, and biological filter) or the number of kilograms of volatile suspended solids (sludge digestion tank). The unit of measurement is usually kg/(m3·d). The weight of pollutants that can be removed per unit volume of aeration tank per unit time.

Fr = Ns × Nw, kgBOD5/(m3·d) or kgCOD/(m3·d). Where: Ns is the sludge load, kgBOD5/(kgMLSS·d); Nw is the mixed liquor sludge concentration (i.e., MLSS), g/L or kg/m3.

Ns = (Lq/Nw) × T where: Lq is the pollutant to be removed per unit volume of wastewater, kgBOD5/m3; T is the aeration time (calculated according to the influent volume), d. After simplification, it can be calculated using the following formula: Fr = [(q1-q2) × 24]/1000V where: q1 is the influent concentration, mg/L; q2 is the effluent concentration, mg/L; V is the aeration tank volume, m3. Using the volumetric load to evaluate the actual treatment load of the bioreactor and the advantages and disadvantages of operation and management under the same conditions is relatively simple and intuitive. In the coking system, using the easily detectable COD volumetric load as a comprehensive evaluation indicator is particularly true.

19. Organic load (F/M): This refers to the amount of organic matter that can be removed per unit volume of filter media (or tank) per unit time. It is an important parameter for the design and operation of biological filters (or aeration tanks). There are two ways to express it: ① Based on the amount of organic matter entering the filter; ② Based on the amount of organic matter removed by the filter. The former should indicate the removal efficiency, while the latter is essentially the oxidation capacity. Organic matter can be expressed as BOD5 or COD, so it is also called BOD or COD load, and the unit is g (or kg)/(m3 (filter media) · d) (for aeration tank: g (or kg)/(m3 (tank volume) · d)). When calculating the amount of organic matter, the organic matter in the return flow is generally not included (when using a return flow system).

20. Sludge age (SRT): Sludge age refers to the average residence time of microbial cells in the aeration tank. For activated sludge processes with return flow, sludge age is the time (in days) required for the average renewal of the entire aeration tank sludge.