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Taking samples for bacterial analysis from flooded anaerobic bioreactor

Taking samples for bacterial analysis
from flooded anaerobic bioreactor

Taking a sample for bacterial analysis from deep
in anaerobic bioreactor during re-construction



Microbiology and Metal Removal

Most of the metal removal mechanisms in the engineered wetland system are microbially-mediated. Three functional groups of bacteria were found: sulphate reducing bacteria (SRB), fermentative bacteria, and iron reducing bacteria (IRB). SRB are critical to the operation of the anaerobic bioreactor cells as they generate alkalinity and cause metals to precipitate. Fermentative bacteria produce substrates (electron donors and carbon source) for the SRB. Fermentative bacteria include obligate anaerobes such as the Clostridia and a number of facultative anaerobes. SRB are obligate anaerobes and can use carbon compounds such as formic, acetic and lactic acids, ethanol, and hydrogen as electron donors. Most IRB are facultative and can use ferric iron as a terminal electron acceptor in the absence of oxygen.

The main metal removal mechanism for As, Cd and Pb was the precipitation of these metals as their sulfides.

2CHO + SO4
H2S + 2HCO3-  
Me 2+ S2-

Where CH2O represents the carbon source and Me is used to represent a typical metal cation in mine drainage.

An adequate supply of sulphate is necessary in the system for the reactions to work. The carbon source may be any type of carbonaceous material (e.g., sawdust, wood) submerged in a wetland cell’s water, the decaying roots/detritus of the wetland plants growing in a wetland cell, or a layer of microbially-available carbonaceous material such as municipal compost or pulp mill biosolids in a bioreactor.

The metalloid As was present as an oxy-anion and may behave differently than the metals. Some strains of SRB can reduce both sulfate and arsenate to produce the mineral orpiment (As2S3) (Newman et al., 1997). The solubility of orpiment is controlled by both sulfide concentration and pH. At high pH values, orpiment becomes more soluble. Because the cells also contain calcium carbonate some of the arsenic may also precipitate as calcium arsenate. Preliminary mineralogical work indicates that most of the arsenic is present in the cells as amorphous arsenic sulfides.

Appreciable numbers of all three groups of bacteria were found in AB1 (Table 4), which indicates that the operation of the cell was satisfactory. High numbers of SRB were found in surface sediments, which may indicate the entire cell was anaerobic. AB2 had bacterial numbers that were on average an order of magnitude higher than the first cell. The higher numbers in the second cell may be due to the fact that it has been in operation for three years longer than the first cell and that at least a year is required for bacterial populations to reach optimal numbers. Anaerobic bacteria (SRB and IRB) were also detected in all three HSSF cells, which indicates that these cells also have anaerobic zones. It is likely that metal removal in the HSSF cells may be due to both filtration (removal of suspended solids) and precipitation as metal sulphides.

Bacterial Removal and Production of Sulphides

The effectiveness of the bacterial activity in metal removal can be established by examining the metal removal from the first two cells – the primary bacterial reactors (Table 5). Bacteria are active in the following plant-based cells as well and may serve as the primary metal removal mechanism. The role of plants may be to evapotranspirate clean water, and possibly to sequester metals. Evapotranspiration may also serve an important function by increasing metal concentration for further bacterial treatment.

Levels of As are reduced by 71% in the first anaerobic (limestone drain) cell and the remaining total is reduced by a further 80% in the second anaerobic cell. Total reduction in the two cells is 94%. For Cd the numbers are 61 and 83% respectively with a total reduction of 93%. For Zn, however, the reduction is much less and we can see that the first anaerobic (limestone drain) reduces the level by only 59% with a further 23% reduction in the second anaerobic cell. The two cells combined reduce the Zn levels by a combined 68%.

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