In anaerobic treatment of domestic wastewater, the retention time is maintained for more than 2 hours, the anoxic retention time for more than 2 hours, and the aerobic tank retention time for 6 hours. It seems that the anaerobic and anoxic retention times are for better removal of organic matter. If the organic matter content is low, does the retention time need to be shortened? And if the organic matter content is high, does the retention time need to be extended? What would be the impact if the retention time is too long or too short? It seems that a longer aerobic retention time is to facilitate the growth of nitrifying bacteria for more effective ammonia nitrogen removal. Thank you, experts, for the (popular science explanation)!
Hydraulic Retention Time (HRT) is often overlooked in daily operation management, yet it is an important reference data point, especially for nitrogen and phosphorus removal systems!
1. What is Hydraulic Retention Time (HRT)?
Hydraulic Retention Time (abbreviated as HRT) is a term used in water treatment processes. It refers to the average time that the wastewater to be treated remains in the reactor, i.e., the average reaction time between the wastewater and the microorganisms in the biological reactor.
For biological treatment, the HRT must meet the requirements of the specific process. Otherwise, if the HRT is insufficient, the biochemical reactions will be incomplete, leading to weaker treatment efficiency. Conversely, an excessively long HRT can cause sludge aging in the system.
Table: HRT for Different Wastewater Treatment Processes
When treatment efficiency is poor, the design HRT value can be used for verification. When verifying HRT, the flow rate should include the sludge return flow. If the HRT is too small, the wastewater flow rate should be slowly decreased; if it is too large, the wastewater flow rate should be slowly increased. Note that any changes in wastewater flow rate should be made gradually to avoid imposing a shock load on the system. Given the challenging nature of wastewater treatment, reducing the incoming wastewater flow rate should not be done lightly; adjustments should primarily be made to the return flow rate.
In the conventional activated sludge process, the HRT largely determines the degree of wastewater treatment because it determines the sludge retention time. However, in the MBR (Membrane Bioreactor) process, the separation effect of the membrane completely retains microorganisms within the reaction tank, thereby achieving complete separation of the hydraulic retention time and the sludge age!
2. Calculation of Hydraulic Retention Time (HRT)
There are actually two types of hydraulic retention time in wastewater treatment: one is called the Nominal Hydraulic Retention Time, and the other is the Actual Hydraulic Retention Time!
1. Nominal Hydraulic Retention Time
As the name suggests, this is the calculation based on the definition: the hydraulic retention time equals the effective volume of the wastewater treatment system divided by the influent flow rate.
If the effective volume of the wastewater treatment system is V (m³), and Q is the hourly influent flow rate (m³/h), then the formula for hydraulic retention time is:
`HRT = V / Q`
2. Actual Hydraulic Retention Time
The actual hydraulic retention time refers to the real time wastewater actually remains in the treatment system, and it needs to account for the sludge return flow:
If the effective volume of the wastewater treatment system is V (m³), Q is the hourly influent flow rate (m³/h), and R is the sludge recirculation ratio, then the formula for hydraulic retention time is:
`HRT = V / [(1 + R) Q]`
So, in a nitrogen removal system, is the internal recirculation flow included in the calculation of the actual hydraulic retention time for the anoxic tank? This issue has been debated. Generally, the internal recirculation flow is not included in the formula for the actual HRT of the anoxic tank. Regulations typically only provide a range for the anoxic tank HRT. For the calculation of the anoxic tank HRT, the external recirculation ratio R is included without dispute; it's generally accepted that the effective influent flow rate is (1+R)Q.
Therefore, the anoxic tank HRT is generally considered as HRT = V / [(1 + R) Q].
Regarding whether the internal recirculation flow should be counted for the anoxic tank HRT, from a macroscopic perspective, if the internal recirculation ratio r = 4 or N, we consider that the water is recirculated 4 or N times. So, although the retention time per pass is short, the total time over 4 or N passes adds up to be equivalent, effectively offsetting the influence of the internal recirculation.
Therefore, the internal recirculation flow is not included in the formula.
3. The Role of Hydraulic Retention Time (HRT)
Effect of HRT on Nitrogen Removal
In the A²/O process, under conditions of a sufficiently long HRT, there is good removal efficiency for NH₃-N. If the HRT is too short, the various microbial populations in the reaction tank do not have enough time to grow, sludge is washed out too quickly, and both nitrification and denitrification reactions do not proceed fully. When the HRT reaches a certain value, sufficient for the reactions in each reactor to proceed fully, further increasing the HRT only adds economic burden without providing a more significant improvement in nitrogen removal.
However, research on hybrid MBR processes has indicated that within the tested HRT range (4.97h - 8.70h), the system's TN removal efficiency increased as the HRT decreased. This is because under long HRT conditions, the organic loading rate of the system decreases, which can intensify endogenous respiration of biomass, affect sludge activity, and ultimately reduce the system's pollutant removal efficiency. Reducing the HRT can increase the system's organic loading rate, thereby enhancing the system's denitrification capacity and finally improving nitrogen removal performance.
Effect of HRT on Phosphorus Removal
In the SBR process, HRT has a relatively small impact on PO₄³⁻-P removal efficiency; this process does not show significant removal效果 for PO₄³⁻-P. This might be because denitrifying bacteria and polyphosphate-accumulating organisms (PAOs) are both heterotrophic. Denitrifying bacteria can uptake and utilize VFAs before PAOs for denitrification, and PAOs have stricter requirements for carbon sources than denitrifying bacteria – readily biodegradable organic matter is preferentially used by denitrifying bacteria. This leads to less carbon source being adsorbed by PAOs, correspondingly less VFA, resulting in less PHB (poly-β-hydroxybutyrate) generated under anaerobic conditions. Consequently, the energy required for phosphorus release is relatively reduced.
Research on the A²/O process shows that as HRT increases, TP removal efficiency does not necessarily increase continuously but rather shows a trend of first increasing and then decreasing. When the HRT is 8h, the TP removal efficiency is the highest, indicating the best removal performance. When the HRT increases to 12h, the TP removal efficiency shows a declining trend, and phosphorus removal performance deteriorates. This indicates that a sufficiently long HRT is beneficial for TP removal. However, as HRT increases further, the TP removal rate gradually decreases, which can have an adverse effect on phosphorus removal. This might be because if the HRT is too large, it can lead to sludge