Environmental Biotechnology - Theory and Application

Pollution and Pollution Control 71

Although we have been considering the issue of metal toxicity in relation to
the contamination of land or water, it also has relevance elsewhere and may be of
particular importance in other applications of biotechnologies to environmental
problems. For example, anaerobic digestion is a engineered microbial process
commonly employed in the water industry for sewage treatment and gaining
acceptance as a method of biowaste management. The effects of metal cations
within anaerobic bioreactors are summarised in Table 4.2, and from which it is
apparent that concentration is the key factor.
However, the situation is not entirely clear cut as the interactions between
cations under anaerobic conditions may lead to increased or decreased effective
toxicity in line with the series of synergistic/antagonistic relationships shown in
Table 4.3.
Toxicity is often dependent on the form in which the substance occurs and
substances forming analogues which closely mimic the properties of essential
chemicals are typically readily taken up and/or accumulated. Such chemicals are
often particularly toxic as the example of selenium illustrates.
Often wrongly referred to as a toxic metal, and though it has some metallic
properties, selenium is a nonmetal of the sulphur group. It is an essential trace
element and naturally occurs in soils, though in excess it can be a systemic poison
with the LD 50 for certain selenium compounds being as low as 4 micrograms
per kg body weight.

Table 4.2 The effect of metal cations on anaerobic digestion

Cation Stimulatory Moderately inhibitory Strongly inhibitory

Sodium 100–200 3500–5500 8 000
Potassium 200–400 2500–4500 12 000
Calcium 100–200 2500–4500 8 000
Magnesium 75–150 1000–1500 3 000

Concentrations in mg/l

Table 4.3 Effective toxicity and synergistic/antagonistic relationships

Toxic cation Synergistic Antagonistic

Ammonium-N Calcium Sodium

Magnesium

Potassium

Calcium Ammonium-N Sodium

Potassium Magnesium

Magnesium Ammonium-N Sodium

Calcium Potassium

Potassium – Sodium

Sodium Ammonium-N Potassium

Calcium