Classification of Water Pollutants

Classification of Water Pollutants

• A pollutant may come from a point source or a dispersed source.
• A point source pollutant enters water from a confined source like a pipe or channel.
• Examples of point sources include pipes discharging domestic sewage or industrial wastewater into bodies of water
• Discharges are treatment plant effluents, treated sewage from water pollution control facilities that still contain pollutants.
• A dispersed or nonpoint source is a broad area where pollutants enter a body of water.
• Surface runoff in agricultural areas carries silt, fertilizers, and animal wastes into streams.
• Common dispersed sources include stormwater drainage systems in towns and cities.
• They cannot be collected and removed from the water.
• Control dispersed pollutants by setting appropriate restrictions on land use
• Point source pollutants are easier to manage because they collect at a single location for treatment.
• Point discharges can be easily monitored by regulatory agencies.
• The Clean Water Act (CWA) requires a discharge permit for all point sources

Categories of Water Pollutants

• Pathogenic organisms
• Oxygen-demanding substances
• Plant nutrients like Nitrogen and Phosphorus: are considered domestic sewage
• Toxic organics include surface runoff from agricultural areas or industrial activities
• Inorganic chemicals
• Sediment
• Radioactive substances
• Heat
• Oil, like runoff from roads

Thermal Pollution

• The temperature of water used for cooling can increase by up to 15°C after condensing steam
• The discharge of warm water into a river = thermal pollution
• Warmer temperatures reduce oxygen solubility and increase fish metabolism, changing the ecological balance in rivers
• Thermal pollution can be controlled by passing heated water through cooling ponds or towers after it leaves the condenser
• Cooled water can then be discharged or reused as plant cooling water

Soil Erosion and Sediment Control

• Movement of soil particles by wind or water is called soil erosion
• Eroded soil is a significant environmental problem
• Suspended soil particles reduce sunlight penetration = decreased photosynthetic activity and disrupting stream ecology.
• When water velocity slows, suspended particles settle and deposit as sediment.
• Sediment disrupts the reproductive cycles of fish and other life forms.
• Two types of water-caused soil erosion exist: sheet erosion and stream erosion
• Sheet erosion occurs when raindrop impact and storm runoff carry soil from land areas
• Rainfall intensity, soil texture, slope steepness, and vegetative cover affect sheet erosion
• Stream erosion is the removal of soils from stream beds and banks by fast-moving water
• Streamflow velocity and soil type affect stream erosion
• The Mississippi River transports an average of 1.5 million tons of sediment daily to the ocean.
• Practices include temporary grass cover on exposed soils until permanent seeding or soil stabilization occurs.
• Mulching materials, like woodchips, provide temporary cover on areas with steep slopes and unsuitable soils.
• Diversion channels can reduce open slope length.
• Temporary fences can reduce erosion at construction sites
• Hay bale and gravel filters prevent sediment entry into drainage systems and local streams

Stream Pollution

• Rivers and streams are surface waters that assimilate biodegradable wastes.
• They have self-purification due to waste-assimilative capacity where systems recover from pollution.
• Factors like pollutant strength/volume, stream discharge/flow rate, and water turbulence affect self-purification
• Dilution solves pollution and the constant flushing action of flowing water is involved in waste assimilation
• Oxygen transfer between the air and the water effects waste assimilation
• Reaeration: oxygen transfer to replenish dissolved oxygen (DO)
• Atmospheric oxygen is dissolved at the water surface which replenishes water.
• Nonbiodegradable pollutants are not assimilated naturally
• Fast-flowing, shallow, turbulent streams re-aerate more effectively
• Increased surface area and contact between air and water in turbulent flow benefits re-aeration

Waste Assimilation in Streams

• First step: Physical processes such as dilution and reaeration occurs here
• Second Step: Microorganisms use dissolved oxygen to metabolize organic pollutants and convert them into harmless substances.
• Extent of assimilation must account for the physical dilution effect of waste discharge
• Mixing and dilution start immediately upon a point discharge entering a flowing stream
• Pollutants do not mix thoroughly at the point of discharge except in turbulent streams
• Waste plume forms instead of an immediate mix.
• Mixing zone length relies on channel geometry, flow velocity, and discharge pipe design

Effect of Dilution - Calculating Pollution

• Diluted concentration is calculated using a mass balance equation assuming complete pollutant mix
• Cd= (CsQs + CwQw) / (Qs + Qw)
  • Cd=diluted concentration or temperature
  • Cs = original stream concentration or temperature
  • Cw = waste concentration or temperature
  • Qs = stream discharge
  • Qw = waste discharge
• An example of this calculation can be performed for a municipal sewage treatment plant effluent being discharged into a stream

Thermal Pollution Calculation

• The maximum allowable stream temperature is calculated from the example equation
  • Given a dry weather discharge of 100cfs at 25°C
  • Compute the maximum discharge of cooling water at 65°C
  • Temperature increase legally limited to 2°C
  • The maximum allowable stream temperature is 27°C
  • Solving the initial equation to derive the final waste discharge equation of 5.3cfs

Dissolved Oxygen Profile

• When sewage goes into a stream, dissolved oxygen (DO) is used by microorganisms to metabolize
• Microbes exert a biochemical oxygen demand known as BOD.
• BOD causes the dissolved oxygen level in the stream to gradually drop.
• Sewage discharge causes utilization of dissolved oxygen by microorganisms. These microorganisms metabolize and decompose organic substances.
• Microbes exert a biochemical oxygen demand, or BOD
• The dissolved oxygen level in the stream gradually drops, this is called the stream deoxygenation curve
• While deoxygenation occurs, oxygen dissolves into the water from the air
• Oxygen transfer depends on temperature and the oxygen deficit
• Oxygen deficit: the difference between actual DO concentration and the saturation DO value
• The larger the deficit = faster oxygen transfer rate.
• Stream is called the reaeration curve known as Curve B

Curve C

• At any given time, the DO level in the stream depends on the combined effects of deoxygenation and reaeration.
• The actual DO equals the sum of the DO on the deoxygenation curve plus the DO on the reaeration curve.
• Combined DO versus time graph is curve C, known as the dissolved oxygen sag curve.
• Initially, deoxygenation exceeds reaeration, so the oxygen profile sags
• The rate of reaeration dominates after most organics decompose which causes the oxygen profile begins to rise
• The minimum dissolved oxygen content in the stream occurs = the rate of reaeration/deoxygenation.

Complete DO Depletion

• Sag curve intersects the horizontal axis at DO = 0, resulting in anaerobic/septic conditions.
• Anaerobic conditions = obnoxious odors and very unsightly conditions in the water
• Eventually, with time and distance the water will re-aerate and the quality of the water will be restored
• Streams have zones of pollution, which can be described as four relatively distinct zones
  • Zone degradation
  • Active decomposition
  • Recovery
  • Clean water

Zone of Degradation

• Is below the point of waste discharge with floating solids, turbidity, and other visual evidence of pollution
• Oxygen levels initially drop in the zone of degradation

Zone of Active Decomposition

• Active decomposition begins when the dissolved oxygen level drops to 40% of saturation
• The water is heavily polluted and higher forms of aquatic life die or migrate.
• Tolerant fish like catfish may survive and the different species mixture is caused by low DO levels

Zone of Recovery

• Reaeration exceeds deoxygenation and oxygen levels increase 40%
• Water is gradually clearing with no odors and desirable aquatic animals reappear

Zone of Clean Water

• Characterized by clear water, high DO, diverse organism thriving, and stables nutrients

Computation of Minimum DO

• Using the Streeter-Phelps equation it is important to predict the minimum DO level in a pollutes system.
• The processes taking place are deoxygenation from BOD and reaeration by oxygen transfer at the surface.
• The minimum DO in the stream is the difference between DO saturation and critical oxygen deficit
• Critical time tc and the critical ocygen deficict Dc is found using equations as outlined
• A practical example is BOD1 in a stream is 3mg/L and the DO is 9mg/L and a formula calculation is shown

Lake Pollution

• Plant nutrients are more important than organics from sewage.
• Phosphorus and nitrogen are the most critical plant nutrients
• Accumulation of pollutants containing leads to the accumulation of phosphorus and nitrogen compounds.
• Eutrophication: Nutrient enrichment and gradual filling in of a lake, a process inevitable for aging.
• The life of a lake has four stages

Lake Pollution Stages

• Oligotrophic: Deep, clear, cold nutrient-poor water, with little aquatic life
• Mesotrophic: Nutrients and sediment begin to accumulate with increased populations of aquatic life
• Eutrophic: Nutrient-rich, shallow, warmer water, with much plant growth and frequent algal blooms
• Senescent: Very shallow, overgrown emergent plants
• Wastewater effluent and runoff containing phosphorus easily trigger algae blooms
• Advanced sewage treatment may get rid of much of the phosphorus and nitrogen, it is an expensive means of control.
• Lakes start our as deep, could, clear bodies know as oligotrophic lakes.
• Silty sentiments begin to from in mesotrophic lakes.
• Eutrophic Stage: relatively shallow and warmer body of water, with enough nutrients to support large populations of plants and animals.
• In eutrophic lake frequent algae blooms.
• Further aging is called a senescent lake with thick deposits of organic silts and very high nutrient levels

Thermal Lake Stratification

• Happens due to temperature different in the water during summer and supports growth of algae
• Lake water is warmed through the air which and the forms the top layer, the epilimnion
• The hypoliminion are colder, denser water that remains at the bottom of the lake.
• The thermocline is a layer of water in which temps rapidly decrease, separating the epilimnion and hypolimnion layers.
• The thermocline acts as a physical barrier for the waters between top and bottom layers of the lake
• As air temperature decreases, epilimnion waters cool, become denser, and sink = fall overturn
• Eventually the lake becomes completely mixed and layers disappear, in a circulation known as fall overturn
• During the cold winter months, ice covers the lake with winter stagnation
• With spring the ice starts to melt as aids helps entire lake mixes
• Oxygen depletion and anaerobic conditions cause lake bottom to have a decrease in oxygen
• Compressed is diffused through tubes placed at bottom of lake to increase oxygen in the water
• One measure is to pump water from out side to control lake conditions