Total Organic Carbon (TOC): TOC is the amount of total organic carbon present in an organic compound except for CO2, carbonates and bicarbonates. It is a general indicator of the approximate level of organic contamination and water quality. It determines the number of carbon-containing compounds in a source. This is important because knowing the amount of carbon in a water stream is an indicator of the organic character of the stream. The carbon-containing compounds can be present in different forms and the proportions of each can be critical. The larger the carbon or organic content, the more oxygen is consumed. A high organic content means an increase in the growth of microorganisms which contribute to the depletion of oxygen supplies. Most organic carbon in water supplies is natural in origin and is derived from living and decaying vegetation. These compounds may include humic and fulvic acids, polymeric carbohydrates, proteins, and carboxylic acids. The presence of excess concentration of TOC can create unfavorable conditions for aquatic life, such as the depletion of oxygen and the presence of toxic substances. There are number of conventional methods of removing TOC such as activated carbon filtration, UV disinfection, reverse osmosis, water distillation, coagulation-precipitation, oxidation or sequestration by adsorption. The most common method is activated carbon filtration. While working with a large waterworks treating surface water activated carbon might be costly and a pretreatment by coagulation-precipitation might be needed. Disinfectants like chlorine are also used to treat water. However, use of chlorine can result in formation of disinfection byproducts (DBPs) through chemical reaction between chlorine and TOCs. TOCs are precursors of DBPs. Biological growth Biological growth can be defined as the proliferation of microbes when resources supporting growth are in abundance. Excess of nutrients such as carbon and nitrogen in water can result in exponential growth of microbes. Apart from nutrients, biological growth can be affected by pH, temperature, oxygen levels, water activity and toxins. The common processes for removal of microbes from water include pretreatment, coagulation/ flocculation/sedimentation, and filtration. Roughing filters, microstrainers, offstream storage, or bank infiltration can be used for pretreatment processes. Filtration can be accomplished using granular media filters, slow sand, precoat filters, membranes, or other filters. Oxidants may be added to water for a variety of purposes, including control of taste and odor compounds, removal of iron and manganese, Zebra Mussel control, and particle removal, among others. Non oxidizing biocides are also applied to kill microbes through interference in the reproduction, ceasing respiration and cell wall lysis. Effect of organic matter in biological growth Organic compounds, either dissolved or particulate (which can be broken down to a soluble form), provide energy sources for heterotrophic bacteria, which use organic carbon to produce new cellular materials or as energy. A portion of TOC is readily available to microbes that helps in proliferation and growth. The biodegradable organic matter (BOM) that is not removed during water treatment can potentially lead to the multiplication of bacteria along the distribution system, which deteriorates the water quality, accelerates corrosion rates of pipes, and can potentially increase the incident of bacteriological diseases. Swati Sharma, PhD Wastewater Technical Consultant 402.624.2286 (O) 402.521.4161 (C) 402.624.2287 ssharma@h2oeng.com