Bio/chemical degradability of coolants

Modern coolants contain one or two base fluids, depending on the need for protecting an engine from freezing temperatures. Hence, if cold weather protection is necessary freezing point depressants are added to water. The main freezing point depressants used in engine coolants are monoethylene glycol (MEG) and monopropylene glycol (MPG), however various other chemicals have been used in the past for engine antifreezes and are still in use in heat transfer applications.

Traditional glycol- and ethanol-based heat transfer products are biodegradable by more than 97% in under five days. This ready biodegradability at the same time can lead to a high oxygen demand which means that they are likely to have adverse impacts if released untreated into the environment. Several tests are available to measure the potential impact of discharges into receiving waters, including the BOD and COD test methods.

BOD is the Biochemical Oxygen Demand, which means the amount of oxygen which is needed to decompose organic matter in a body of water by bacterial degradation. The more oxygen used during decomposition (for example, in sewage or heavily polluted water), the less is available for fish and other aquatic life. This may lead to oxygen starvation of the water course and the various aquatic species. Hence, wastewater is often treated before being discharged, in part to lower the BOD, which is dependent on the consents and permits in place and the capability of the site. Testing the BOD value of a water sample takes five days and uses the introduction of certain specified bacteria to represent what would happen in nature.

COD is the Chemical Oxygen Demand, a measure of how much oxygen would be needed to completely oxidise the carbon in a sample. A strong chemical agent is introduced into the sample for the COD test, which takes two or three hours. The strong oxidising effect of the reagents in the COD test gives a more complete conversion of the substance tested and can be thought of as a maximum amount of required oxygen for chemical conversion. The BOD test, however, gives an idea of how much oxygen would be required under normal biochemical (bacteriological) conditions.

Most pristine rivers will have a 5-day BOD below 1 mg/l. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/l. Rivers may be considered severely polluted when BOD values exceed 8 mg/l. Municipal sewage would have a BOD value of about 20 mg/l or less, while untreated sewage varies, but averages around 600 mg/l in Europe.

From an oxygen-depleting standpoint, it is important to note that the oxygen demand for biodegradation of MPG is substantially larger than that of MEG and may have a larger, negative impact on the flora and fauna present in soil and water. As the oxygen demand in the biodegradation process for MEG is lower than for MPG, wastewater treatment plants can handle MEG more easily than MPG.

So, most base fluids in coolants / antifreezes like MPG, MEG, ethanol, diethylene glycol, propionate, etc. are biodegradable and will soon break down into carbon dioxide and water. Neither should be dumped in the environment as they contribute to water pollution, contain chemical additives, and have been contaminated with trace metals and oil in use. Coolants / antifreezes should be returned to a recycling centre to minimize harmful effects on our environment after use and spills should be cleaned up immediately.