Page 254 - Hydrogeology Principles and Practice

HYDC06 12/5/05 5:34 PM Page 237

Fig. 6.23 Diagrams showing (a) a
schematic cross-section of a conventional
septic system, including septic tank,
distribution pipe and groundwater plume
and (b) the sequence of simplified redox
reactions in the two principal zones of a
conventional septic system, the septic tank
and drain field. After Wilhelm et al.
(1994b).

Table 6.8 Hydrochemical and
Reaction Equation
biogeochemical reactions in septic
systems. After Wilhelm et al. (1994b).
Anaerobic zone (septic tank and biological mat)
Organic molecule hydrolysis: (1)
Proteins + H O → amino acids
2
Carbohydrates + H O → simple sugars
2
Fats + H O → fatty acids and glycerol
2
Ammonium release:
+
+
Urea [CO(NH ) ] + H O → 2NH + CO 2 (2a)
3 2
2
4
+
Amino acids + H O → NH + organic compounds (2b)
4
2
Fermentation:
−
Amino acids, simple sugars → H , acetate (CH OO ), other organic acids (3)
2
3
Anaerobic oxidation:
Fatty acids + H O → H , CH OO − (4)
2
3
2
Sulphate reduction:
+
2−
SO + 2CH O* + 2H → H S + 2CO + 2H O (5)
2
2
4
2
2
Methanogenesis:
+
−
CH OO (acetate) + H → CH + CO 2 (6a)
3
4
CO + 4H → CH + 2H O (6b)
2
2
2
4
Aerobic zone (unsaturated zone and saturated zone to lesser extent)
Organic matter oxidation:
CH O + O → CO + H O (7)
2
2
2
2
Nitrification:
+
−
+
NH + 2O → NO + 2H + H O (8)
3
2
2
4
Sulphide oxidation:
2−
H S (or organic sulphide) + 2O → SO + 2H + (9)
2
2
4
Carbonate buffering:
−
+
H + HCO → H CO 3 (10a)
2
3
+
2+
CaCO + H → Ca + HCO − 3 (10b)
3
2+
CaCO + CO + H O → Ca + 2HCO 3 − (10c)
2
2
3
Second anaerobic zone (saturated or near-saturated conditions)
Denitrification:†
+
−
4NO + 5CH O + 4H → 2N + 5CO + 7H O (11)
2
2
2
2
3
* Organic matter is simplified as CH O throughout. Actual organic matter contains C of
2
various oxidation states and other elements such as N, P and S, and therefore actual reaction
products vary.
† Nitrate reduction can also be accomplished via oxidation of reduced sulphur compounds.