The treated water at the outlet of treatment plants and representative service reservoirs of Mumbai city have been evaluated for trihalomethane formation potential in 1995-1996. Chloroform, dichlorobromomethane, chlorodibromomethane and bromoform have been monitored during monsoon, winter and summer. The levels of chloroform are found above the regulated WHO guideline value of 200 microg L(-1) in final water during postmonsoon at Ghatkopar (226 microg L(-1)), Malbar (210.3 microg L(-1)) and Tulsi (231.26 microg L(-1)).

Trihalomethanes (THMs) the by-products of chlorination in water treatment are recognised as a threat to public health due to their carcinogenicity. The photodegradation of THMs using hydrogen peroxide has been found to give increased removal efficiency and the outcome of the study may find, its application in designing a unit process for water treatment. Batch experiments were carried out using UV lamp of 83 W and 40% w/w Hydrogen peroxide (H(2)O(2)) in test waters between 2.5-10 pH range of chloroform, bromodichloromethane, dibromochloromethane and bromoform at 50-200 microg L(-1) initial concentration. 92-100% removal of chloroform, bromodichloromethane, dibromochloromethane and bromoform were found with 0.1% of H(2)O(2) and 90 min of UV exposure.

The formation of methyl nitrite was detected when two clay soils were treated with nitrous acid (260 ± 20 mg N kg−1 soil) in the dark in a closed chamber. The presence of the gas was positively identified by mass spectrometry. There was a consistent trend with time in the concentrations of methyl nitrite in the atmospheres above the soils. A rapid buildup occurred following addition of nitrous acid, but this was soon followed by a gradual decline in concentration. The peak concentrations occurred approximately 15 min or 30 min after nitrous acid addition, and 5‐h concentrations were < 50% of peak concentrations. Methyl nitrite was formed under mildly to moderately acid conditions (pH 6.3 and 4.8) at −33 kPa soil water potential. The results indicate that methyl nitrite may form under conditions encountered in the field, and that soil can be a sink as well as a source of methyl nitrite.

The mass-transfer coefficient of a free-fall cascade-aerator unit of 15 million litres per day was evaluated for its efficiency in the removal of a class of volatile organics, the trihalomethanes (THMs). These compounds are carcinogenic and occur as a result of chlorination of natural waters. Due to the volatile nature of the THMs, the efficiency of aeration as a potential technique for their removal has been studied. The principle behind aeration is gas-transfer, according to which the gas-liquid interface is hypothesized to consist of a gas and liquid film through which gas is transferred by molecular diffusion until equilibrium is attained. The overall mass transfer coefficient (K(L)) of the aerator considering oxygen as the reference compound, was found to be 29.3 hr(-1) for THMs.