Notes for Water Purification Processes in Natural Systems.pdf

WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ Natural forms of pollutants have always been present in surface waters. Many of the impurities were washed from the air, eroded from land surfaces or leached from the soil and found their way into surface water. Natural purificatio...

WATER PURIFICATION PROCESSES IN NATURAL SYSTEMS ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ Natural forms of pollutants have always been present in surface waters. Many of the impurities were washed from the air, eroded from land surfaces or leached from the soil and found their way into surface water. Natural purification processes were able to remove or otherwise render these materials harmless. Human activity increased the amount and changed the nature of pollutants entering watercourses Settlements → Villages → Towns → Cities Quantity of waste products increased until the self – purification capacity of local bodies of water was exceeded. Smaller streams were first affected then larger streams and lakes ultimately becoming polluted. Only in recent decades have POLLUTION CONTROL PROGRAMS been initiated in an attempt to reduce contaminants discharged to bodies of water to the level that the natural purification processes can once again assimilate them. Self –purification mechanisms of natural water systems include: physical, chemical, and biological processes. Speed and completeness with which these processes occur depend on many variables that are system specific. System variables that have an influence on the natural purification process are: a. hydraulic characteristics b. physical characteristics of bottom and bank material c. variations in sunlight d. Temperature e. chemical nature of the natural water The same physical, chemical and biological processes that serve to purify natural water systems also work in engineered systems. In water and wastewater treatment plants, the rate and extent of these processes are managed by controlling the system variables. A thorough knowledge of the natural purification processes is essential to the understanding of 1) the assimilative capacity of surface waters ‒ refers to the amount of contaminant load that can be discharged to a specific water body without exceeding the water quality guidelines. 2) the operations of engineered systems PHYSICAL PROCESSES INVOLVED IN THE SELF - PURIFICATION OF WATERCOURSES 1) Dilution ▪ ▪ ▪ Wastewater disposal practices were based on the premise that “the solution to pollution is dilution” It was considered the most economical means of wastewater disposal and was considered good engineering practice Although a powerful adjunct to self – cleaning mechanisms of surface water, its success depends upon discharging relatively small quantities of waste into large bodies of water Water Purification Processes in Natural Systems │ 1 Prepared by: Engr. MAAbellera ▪ ▪ ▪ Growth in population and industrial activity, with increasing water demand and wastewater quantities precludes the use of many streams for dilution of raw or poorly treated wastewaters Under present regulations, maximum allowable loads are set independently of dilution capacity – only when the standard maximum load is violated then dilution capacity is considered. The dilution capacity of a stream can be calculated using the principles of mass balance. If the volumetric flowrate and the concentration of a given material are known in both the stream and waste discharge, the concentration after mixing can be calculated as: CsQs + CwQw = CmQm where: C – the concentration of selected material (in mass/volume) Q – the volumetric flowrate (volume/time) s,w and m – means stream, waste and mixture conditions SAMPLE PROBLEMS 1. A treated wastewater enters a stream as shown. The concentration of sodium in the stream at point A is 10 mg/L and the flowrate is 20 m3/s. The concentration of sodium in the waste stream is 250 mg/L and the flowrate is 1.5 m3/s. Determine the concentration of sodium at point B assuming complete mixing has occurred. 2. 2. Clean Creek is flowing at an average rate of 0.5 m3/s. Dirty Corps is importing 30% of the creek water to be used as wash water which is then discharge again to the creek. The upstream of Clean Creek has 20 mg/L of Ca2+ and Dirty Corps discharge has seven times as much. Calculate the concentration of calcium ion after the mixing point. 3. 3. Fifty cubic meters of water containing 28 mg/L solids passes the Gallo River in one minute. There is a city ordinance limiting the solids content of the river to 40 mg/L. If the Olio Corps will be discharging water with 86mg/L solids to the river, determine the maximum rate that the company can discharge without being penalized. 4. 4. After the rain, Muddy Creek became muddy flowing with 136mg/L of solids at 5.1 m3/s. The creek is connected to Unyielding River which has 27mg/L of solids after that rain and is flowing at 52m3/s. What is the solids concentration a kilometer downstream their mixing point assuming 10% of solids settled in the process? 5. 5. Effluent from a wastewater treatment is discharged to a surface stream. The characteristics of the effluent and stream are as follows: Determine the stream characteristics after mixing with the waste has occurred. Water Purification Processes in Natural Systems │ 2 Prepared by: Engr. MAAbellera 2) Sedimentation and Re – suspension Suspended solids are one of the most common water pollutants and in suspension, solids increase turbidity and reduce light penetration may restrict the photosynthetic activity of plants, inhibit vision of aquatic animals, interfere with feeding of aquatic animals that obtain food from filtration and be abrasive to respiratory structures such as gills of fish. ▪ Sedimentation ‒ nature’s method of removing suspended particles from a watercourse and most large solids will settle out readily in quiescent water ‒ Particles in the colloidal size range can stay in suspension for long periods of time though eventually most of these will also settle out. ‒ This natural sedimentation is not without drawbacks. Anaerobic conditions are likely to develop in sediments and any organics trapped in them will decompose, releasing soluble compounds into the stream above. ‒ Sediments deposit can also alter streambed by filling up the pore space and creating unsuitable conditions for the reproduction of many aquatic organisms. ‒ It can also alter its course or hamper navigation activities and it reduce reservoir storage capacities and silt in harbors and increase flooding due to channel fill – in. ▪ Resuspension of solids is common in times of flooding or heavy runoff. Increased turbulence may resuspend solids formerly deposited along normally quiescent areas of stream and carry them for considerable distances downstream and eventually they will settle again. 3) Filtration ▪ ▪ ▪ ▪ Large bits of debris lodge on reeds or stones as they move along streambeds, and they remain caught until high waters wash them into mainstream again. Small bits of organic matters and inorganic clays and other sediments may be filtered out by pebbles or rocks along the streambed. A water percolates from the surface downward into groundwater aquifers, filtration of much more sophisticated type occurs. If the soil layers are deep and fine enough, removal of suspended material is essentially complete by the time waters enter the aquifer. 4) Gas Transfer ▪ The transfer of gases into and out of water is an important part of the natural purification process. ▪ The replenishment of oxygen lost to bacteria degradation of organic waste is accomplished by the transfer of oxygen from the air into the water. ▪ Conversely, gases evolved in the water by chemical and biological processes may be transferred from the water to the atmosphere. ▪ Gas transfer is affected by solubility (extent to which gas is soluble in water) and transfer rate (rate at which dissolution or release occurs) Water Purification Processes in Natural Systems │ 3 Prepared by: Engr. MAAbellera 5) Heat Transfer ▪ ▪ ▪ ▪ Bodies of water lose and gain heat much more slowly than do land or air masses and under most circumstances, water temperature is fairly constant and changes gradually with the seasons. Meteorological variables and other factors such as channel characteristics (depth, width, surface area), channel volume etc. affect the rate of heat transfer in bodies of water. For streams heated by solar radiation over several miles of heat. Aquatic plants and animals have not developed sufficient adaptability to deal with abrupt changes in temperature and only the most hardy species survive such changes. CHEMICAL PROCESSES INVOLVED IN THE SELF - PURIFICATION OF WATERCOURSES ▪ Natural watercourses contain many dissolved minerals and gases that interact chemically with one another. ▪ Redox (reduction – oxidation), dissolution – precipitation and other chemical conversions may alternately aid or obstruct natural purification processes in natural water systems Chemical Conversions: 1. Oxidation – reduction conversion – biochemically mediated 2. Dissolution – precipitation ‒ solid dissolve in water is essential to the metabolic and reproductive activities of microorganisms that degrade and stabilize organic waste – this is directly or indirectly influenced by dissolution – precipitation. 3. Natural chemical conversions that take place in water can change materials into a form that is soluble and therefore usable by various aquatic organisms. Example: N and P – most essential nutrients for the growth of microorganisms and plankton. 4. Chemical conversions can help stabilize pH of water bodies. Example: HCO3- acts as a buffer to protect a stream from pH fluctuations harmful to aquatic systems. BIOLOGICAL PROCESSES INVOLVED IN THE SELF - PURIFICATION OF WATERCOURSES ▪ ▪ Chemical reactions are biologically mediated – these reactions are not spontaneous and require external sources of energy for initiation. Metabolism – sum total of the processes by which living organisms assimilate and use food for subsistence, growth and reproduction. Metabolic processes and the organisms involved are a vital part in self – purification of natural water system. Types of Metabolic Processes: 1. Catabolism ‒ provides the energy for the synthesis of new cells, as well as for the maintenance of other cell functions Water Purification Processes in Natural Systems │ 4 Prepared by: Engr. MAAbellera 2. Anabolism ‒ provides the material necessary for cell growth ‒ When external food source is interrupted, organisms will use stored food for maintenance energy – a process called Endogenous catabolism. Microorganisms that play an important role in Natural Water Systems 1. Bacteria – the primary decomposers of organic material. They are classified according to the energy and material sources that they require: a. Autotrophs – organisms that derive both energy and material from inorganic sources. Their major function is to convert N and S compounds into stable end – products b. Heterotrophs – bacteria that obtain both energy and material from organic sources. Most important bacteria in the degradation of organic material. They are further classified into: ✓ aerobic heterotrophs – require O2 in their metabolic process ✓ anaerobic heterotrophs – utilize organics in the absence of O2 ✓ facultative heterotrophs – functions as aerobes when O2 is present and anaerobic in the absence of O2 ✓ Phototrophs – utilize sunlight for energy and inorganic substances for material source 2. Algae – these are autotrophic, photosynthetic organisms which metabolize the waste product of heterotrophic bacteria while obtaining energy from sunlight. 3. Protozoa – single – cell organisms that reproduce by binary fission. Protozoa are voracious consumers of organic material and are important members of the aquatic community. Other organisms: Rotifers and crustacea, sludge worms, etc, Water Purification Processes in Natural Systems │ 5 Prepared by: Engr. MAAbellera