Course: BFM 203 Water Treatment and Control

Lecture

Factors Affecting Water Quality

Factors Affecting Water Quality

Pollution (such as acid rain) from our use of the environment impacts water quality, as do activities within the drainage area of a reservoir. Drainage area issues include how wastewater is handled, agricultural runoff, drainage from mining areas, runoff from urban areas, and industrial discharges. Natural factors also impact water quality, including climate (temperature, wind, and precipitation), watershed and drainage areas (geology, topography, vegetation, and wild animal use), wildfires, and the reservoir area itself (geology, depth, area covered, bottom topography, and plants in existence before the reservoir was filled). Point source contamination must be considered when establishing a list of factors that affect each watershed.

Reservoirs are classified in one of three categories depending on the nutrient level present. At the upper end of the nutrient scale we have eutrophic reservoirs that are rich in nutrients; therefore, they support much plant and aquatic animal life. Oligotrophic reservoirs are at the other end of the spectrum; they are poor in nutrient levels, supporting little plant or aquatic animal life. Mesotrophic reservoirs fall in between eutrophic and Oligotrophic reservoirs, hosting a moderate nutrient level and supporting some plant and animal aquatic life. Reservoirs usually start life as Oligotrophic, move through Mesotrophic, and eventually become Eutrophic if conditions are right for aquatic plant and animal growth. Shale, lime, and gravel reservoirs usually do not support this process and may remain Oligotrophic.

Nutrient Levels in Reservoirs

Reservoirs are classified into one of three categories based on the level of nutrients present.

An "Oligotrophic" classification means that few nutrients are present in the water; therefore there is little plant or aquatic animal life.

A "Mesotrophic" classification means that that moderate nutrients are present in the water. A reservoir classified as Mesotrophic supports some plant and aquatic animal life.

A reservoir classified as "eutrophic" is rich in nutrients. Therefore, these reservoirs support much plant and aquatic animal life.

Water quality problems can also be caused by various nutrients in our reservoirs and lakes. Nutrients such as phosphate, nitrate, and organic nitrogen compounds act as a fertilizer in lakes to stimulate algae growth. Algae growth in reservoirs is mainly a nuisance, but it can can cause taste and odor problems and adversely affect the reservoir as the plant life dies off.

Algal Blooms. Approximately 40 types of algae have been identified as taste and odor producers; the taste/odor has been described as fishy, aromatic, grassy, septic, musty, and/or earthy. Unpleasant taste/odor is more noticeable when water is warmer. Algal blooms cause taste and order problems for consumers, but chlorination can decrease or increase taste and odor problems. Conventional treatment plans can generally control taste and odor when operated properly because algal blooms and the subsequent die off is a recognizable factor that can be anticipated. Reservoir turnover, however, may happen suddenly and catch treatment plant operators off guard. When turnover occurs, all intake levels may provide unacceptable source water. Alternative sources may be required until reservoir quality returns to an acceptable level.

On the treatment side, algal growth may cause a variety of issues. Certain species of algae may clog treatment filters (causing shortened filter runs). Runs may be reduced due to gas released by the algae. More backwashes and shorter runs reduce production and increase costs. Algal blooms also increase pH in the upper layer of the reservoir where blooms occur. pH increases from 7 to 9 are not uncommon. Treatment costs may increase due to pH adjustment chemicals or the use of additional chlorine. Increases in pH are more noticeable during the day and less noticeable at night due to the photosynthetic properties of algae. Respiration results in increased carbon dioxide at night and photosynthesis during the day decreases carbon dioxide. Swings in pH can affect coagulation and disinfection processes. More frequent analysis of source water is necessary during algal bloom and die off conditions to ensure that treatment systems are adjusted as source water characteristics change.

Dissolved Oxygen Depletion

Dissolved oxygen (DO) depletion is a concern with algal growth as well. As algal blooms occur, DO increases due to photosynthesis. When algal blooms die, DO decreases as bacteria metabolize the algae cells. Low levels of sunlight amplify the condition. Extremely low DO can cause fish kills; dead fish in a drinking water reservoir that is visible to the general public can create a lack of public confidence in water safety and in the utility's ability to manage and treat their drinking water. While cleanup of dead fish is very costly, cleanup should occur in a timely manner.

Organic Loading

Organic loading occurs when algal blooms die off, creating an increased level of organic matter in a reservoir. This die-off causes discoloration, necessitating higher levels of chlorine. Organic matter contains THM (Trihalomethanes) precursors; the organic matter reacts with chlorine to create carcinogenic (cancer causing) THMs.The maximum level of acceptable THMs is MCL of 100 ppb in water systems serving more than 10,000 and surface water without filtration. The organic load caused by algal bloom die off can easily exceed the treatment system's ability to chlorinate effectively and of course increases the utility's cost of treating to acceptable standards.

Thermal Stratification

Thermal stratification develops in reservoirs in the spring when surface waters begin to warm. Stratification refers to the layers formed by water of different densities (see illustration below). Stratification is related to turnover (circulation of water from top to bottom). In lakes and reservoirs, turnover occurs when water temperature changes at various depths. Warmer water is less dense than cooler water, so warmer water tends to migrate to the top of the body of water. Lakes and reservoirs will either be classified as monomictic (single turnover annually) or dimictic (two turnovers annually).

In a monomictic reservoir (in deeper lakes that do not freeze over), water is the same temperature from top to bottom during the winter months; thus, the water density is also the same. In the spring, water warms on the surface and winds circulate water in the upper layer only, slowing the mixing action in the reservoir overall. The upper layer (the epilimnion) continues to mix. The middle layer (the metalimnion or thermocline) temperature decreases rapidly with depth. The lowest layer of colder denser water (the hypolimnion) may become anaerobic due to excessive biochemical oxygen demand (BOD).

Source: http://profmandia.wordpress.com/2010/08/16/climate-change-impact-on-freshwater-wetlands-lakes-rivers/

In a dimictic reservoir, two turnovers occur annually; the second turnover is due to freezing of surface water.

Stratification does not normally make water unacceptable for treatment. Destratification, however, may mix the anaerobic water and silt throughout the entire reservoir. This excess turbidity and BOD make treatment difficult or impossible.

Anaerobic Conditions

Anaerobic conditions may be the result of a reduction in the amount of oxygen, an increase in the amount of hydrogen, or the addition of electrons to an element or compound. Under anaerobic conditions, sulfur compounds are reduced to hydrogen sulfide (H_2S). Hydrogen sulfide causes a strong odor in the water, making it unpleasant to the customer.

Anaerobic conditions may also result in reverse behaviors (addition of oxygen, removal of hydrogen, or removal of electrons from an element or compound). Iron and manganese in anaerobic water are oxidized after entering the distribution system. Iron and manganese that come out of solution in the distribution system cause stains on plumbing fixtures and laundry.

Similar to the dead fish situation previously discussed, color and odor issues lead to a lack of public confidence in the ability of a utility to provide safe, clear drinking water.

Water Shed Conditions

Seasonal runoff may cause turbidity and nutrient loading problems. Increased turbidity causes shorter filter runs, reduced flow rate through filters, and increased chlorine demand. Nutrient loading may cause algal blooms and contribute to organic loading and the associated THM precursors noted previously. Keeping records related to reservoir water quality and effective treatment system changes allows operators to plan for these changes each year.