Wastewater: Activated Sludge – Key Control Parameters can save a lot of time and money
How often has been found that insufficient data has been available relating to key control parameters of wastewater treatment operation and performance? Unfortunately the answer is “way too often”.
- Many business could save themselves a lot of time and money through recording and reviewing key control parameters on a regular basis.
The establishment of target and optimum ranges for key control parameters can assist in forecasting when non-compliances could arise and allow for suitable remedial measures to be implemented before a problem develops.
Key control parameters relating to the Activated Sludge Process include:
BOD (or COD) load: Units: kg/day
- Biological wastewater treatment plants are designed and operated on the basis of the oxygen demand (BOD or COD) received and removed.
- The concentration of BOD (in mg/l) in the influent can be used to calculate the total BOD load per day being treated. This is simply done by multiplying the BOD in mg/l by the daily effluent volume in cubic metres (m3) and dividing the product by 1000.
Mixed liquor suspended solids (MLSS):Units: mg/l
- This parameter is essential for the calculation of the F/M loading (and sludge age).
- Sometimes the alternative parameter ‘mixed liquor volatile suspended solids’ (MLVSS) is used. This refers to the ‘volatile’ or organic fraction of the MLSS.
F/M loadingUnits: kg BOD/kg MLSS/day
- ‘Food to Mass’ ratio determines the degree of BOD removal likely to be achieved.
- In general the lower the F/M loading the greater the BOD removal efficiency.
- If the F/M ratio becomes too low, certain operating problems can occur.
Sludge age (Mean Cell Residence Time): Units: days
- The sludge age or Mean Cell Residence Time (MCRT) may be defined as the mass of solids (MLSS) in the plant at any time divided by the mass of new solids made each day.
- The higher the F/M loading the shorter the sludge age and vice versa.
- This is because new biomass is produced at a fast rate when food supply is high and at a slower rate as food supply is reduced.
- A long sludge is required for certain specific objectives, nitrification being the most usual.
- Solids- liquid separation problems can result if the sludge age is excessively long.
Dissolved oxygen: Units: mg/l
- The dissolved oxygen should be maintained above 1 – 1.5 mg/l in all parts of the aeration basin.
- Where nitrification is required the dissolved oxygen needs to be 2 mg/l or higher in all parts of the basin.
- Reduced dissolved oxygen is a key indicator of treatment plant loading or performance.
- The principal nutrients which often have to be supplied in chemical form are nitrogen and phosphorous. The usual minimum requirement is related to the BOD of the raw wastewater (influent), ie BOD:N:P.
- A range of micronutrients (e.g. trace metals such as copper, manganese, cobalt, selenium etc.) may also need to be supplied.
Sludge volume index (SVI): Units: ml/g
- SVI is calculated as
- Settleability (ml/l) / MLSS (g/l)
Excess sludge production: Units: kg/d
- A record should be kept of the quantity of excess sludge solids removed from the plant each day.
Upward Flow velocity (clarifier): Units: m/h
- For effective solids-liquid separation in a clarifier (settling tank) it is necessary that the rate of rise of the liquid (the upflow velocity) should be significantly less than the natural settling velocity of the solids.
- If this is not the case there will be carryover of solids with the final effluent discharge.
- Upward Flow velocity is calculated by dividing the total flow through the tank (m3/h) by the surface area of the tank (m2). The result is expressed in m/h.
- The desirable upward flow velocity depends on the nature of the particles to be removed.
For more information please contact ECOS Environmental Consultants Limited at 061 email@example.com.