The major micronutrients are nitrogen and phosphorus in activated sludge process. In general, the ratio of BOD : TKN : TP is recommended as 100:5:1 in raw wastewater, but the ratio varies somewhat depending on SRT and other process conditions. Deficiency of one of the two major micronutrients can lead to high membrane fouling due to viscous bulking, where extracellular polymeric substance (EPS) level in mixed liquor rises (Jenkins et al., 2004). If the mixed liquor is observed through microscope after staining with India ink, polysaccharides lumps appear white spots as shown in Fig.1. (Note: India ink preferentially stain hydrophilic surface, but polysaccharides lumps have hydrophobic surface)
Fig. 1. Polysaccharides lumps observed through a microscope at 100x after dying with India ink.
Minor micronutrients such as Ca, Mg, Fe, Mo, Co, Ni, Se, etc. play important role in activated sludge process: balancing osmotic pressure in and out of cells, being an active site in enzyme as a cofactor, being a counter ions for biomolecules, etc. It has been known that high divalent ion concentration and high di- to monovalent ion ratio tend to improve sludge settling properties by increasing average floc size (Park, 2002). It has been also known that some transitional metals, e.g. Mo, Co, Ni,etc, can improve microbial activity in certain environment (Burgess, 2000).
However, the effect of minor micronutrient on membrane fouling remains still fuzzy as follow.
- Table 1 shows elemental composition of healthy activated sludge. It might be plausible to feed all the nutrient proportional to the composition in the table. However, it is not clear whether microorganisms actually need such elements at such levels or they just uptake the metals since they are available.
- Lab scale data reveals some transitional metals such as Co, Mo, Ni can either positively or negatively affect biological BOD degradation (Burgess, 2000). However, no explanation can be given why a few ppm level of cobalt, molybdenum,etc. were required while only extremely small amounts of such metals are required as cofactors of enzyme.
- There are many incidents that wastewater with trace metals at below detection limits run without issue.
- There are no strong evidences that show the crucial role of minor micronutrients in long-term full scale activated sludge operation. The inorganic compositions of various mixed liquor samples are summarized in Table 2. Though all plants were running without a significant issue, but minor micronutrient contents vary in wide range.
Table. 1. Composition of activated sludge (Metcalf&Eddy, 2003)
|Range, %||Representative, %|
Table 2. Various compositions of mixed liquor taken from the MBR in different locations.
|Location (All are municipal MBR except B)|
* Mixed liquor was digested before performing ICP analyses. Elemental compositions are not normalized to MLSS, but all samples had 2.5-4.0 g/L MLSS.
© Seong Hoon Yoon