In recent decades, concerns over trace organics such as pesticides, pharmaceuticals, personal care products, steroidal hormones have been growing. Many of those trace organics are suspected as endocrine disruptors on aquatic organisms and eventually on human (Yoon, 2007).
Though exact removal mechanism by activated sludge is still under investigation, following mechanisms are plausible. However, it is not readily possible to accurately split the contributions of each mechanism due to the complex interaction among the different mechanisms and other environmental parameters.
Trace organics such as pesticides, pharmaceuticals, personal care products, steroidal hormones are generally either slowly biodegradable or not biodegradable at all. For biodegradable trace organics, it is reasonable to postulate that biodegradation rate is mainly affected by the adaptability of microorganisms and/or the level of enrichment of the naturally occurring microorganisms specific to certain organic species. Under this logic, longer solids retention time (SRT) is important . Since MBR runs at substantially longer SRT than conventional activated sludge (CAS) process as discussed here, MBR has been considered better for trace organic removal. .
In one study performed by MBR with municipal wastewater, the removal efficiency of phenazone, propyphenazone, and formylaminoantipyrine (FAA) increased from 10-35% to 60-70% over 5 month period of pilot test (Zeuhike, 2007). In the same period, removal efficiency of the same trace organics by CAS remained consistently in 15-40%. On the contrary, removal efficiency of estrone and estradiol was about same in both MBR and CAS. These observations suggest that biodgradation is not the sole mechanisms with which trace organics are removed.
- Adsorption on biomass and other suspended solids
For refractory organics, adsorption on biomass and other suspended solids is an important removal mechanism especially for non-biodegradable compounds. Due to the limited microbial adsorption capacity and the competitive adsorption environment for trace organics, following can be deduced.
- The compounds with low polarity (or high hydrophobicity) are subject to be removed by adsorption in addition to biodegradation. As shown
- For a specific compound, removal efficiency tends to decline as initial concentration increases since adsorption capacity may deplete.
- High SRT is not advantageous to obtain high removal efficiency since the amount of sludge removal is low. Therefore, if compounds are not biodegradable at all, removal efficiency of MBR can be lower that that of CAS.
- High SRT is beneficial to enrich proficient microorganisms for slowly degrading compounds, but the lower sludge removal affects removal efficiency negatively by reducing sludge removal.
- If multiple compounds exist, they can compete to be adsorbed. The compound with high affinity is preferentially adsorbed to other compounds. Therefore, the existence and the level of other compounds can affect removal efficiency of a species.
- In order to obtain reliable removal efficiency, MBR must run for at least 2-3 time of SRT until steady state is reached. For example, if SRT was 20 days, MBR must run at least 40-60 days from the perspective of removal mechanisms by adsorption. Otherwise, the removal efficiency might be overestimated especially for non-biodegradable compounds.
- If adsorption is the major removal mechanism, removal efficiency can decrease over time if adsorption capacity reaches to the maximum within SRT. If the same compound was added slowly, no adverse effect may be observed.
Due to the complex interaction among biological adaptation, biological degradability, adsorption, SRT, temperature, and other environmental factors, it is common to find experimental data is far from literature data (see Table 1 here). Therefore, it is not surprising to see substantially different removal efficiencies from seemingly very similar experimental setups.
- Adsorption on membrane
Adsorption of trace organics on membrane has been known quite some time. In one study, powdered activated carbon and subsequent ultrafiltration (PAC-UF) system was used to remove o-dichlorobenzene (DCB) from water (Kim, 1996). It was observed that significant amount of DCB was adsorbed on membrane, which caused competitive adsorption environment between PAC and membrane. As a result, DCB concentration in UF permeate was much lower than what model predicted.
However, this mechanism would not be significant in the long run because the adsorption capacity of membrane is limited. In addition, the desorbed organics from the membrane during membrane cleaning will go back to mixed liquor eventually (even spent cleaning solution is sent back to MBR), which makes net adsorption by membrane zero.
- Evaporation / Stripping
Volatile trace organics can evaporate from water surface or stripped by bubbling. However, most of trace organics, if not all, are heavier than 100 Da and have functional groups that cause varying degree of polarity. Due to the high boiling point and very low vapor pressure at moderate temperature, it is very unlikely this mechanisms contributes to the removal efficiency meaningfully.
© Seong Hoon Yoon