Wastewater Treatment Lecture 14 PDF

Wastewater Treatment  Nature of Wastewater Domestic wastewater is a combination of human feces, urine, and “graywater” Gray water = water from washing, bathing, and meal preparation How do we treat wastewater?  Based on one major metric: BOD Biological Oxygen Demand: the amoun...

Wastewater Treatment  Nature of Wastewater Domestic wastewater is a combination of human feces, urine, and “graywater” Gray water = water from washing, bathing, and meal preparation How do we treat wastewater?  Based on one major metric: BOD Biological Oxygen Demand: the amount of dissolved oxygen consumed by microorganisms during the biochemical oxidation of organic and inorganic matter. Organics –carbon-based Inorganics-ammonia/N How do we perform the BOD test? BOD5 test: measure of oxygen consumed by a microbial community of heterotrophic bacteria in dark conditions at room temperature over 5 days Dissolved oxygen determined at time 0, and after a 5-day incubation period Carried out on a series of dilutions of incoming wastewater https://www.youtube.com/ watch?v=v33CgWJ2TZ0 How do we perform the BOD test? BOD (mg/L) = D1-D5/P Where, D1 = initial dissolved oxygen (DO; mg/L) D5 = final DO (mg/L) P = decimal volumetric fraction of sample used Why use this test? Determines amount of oxygen needed for biological treatment of organic matter present in wastewater Determines required size of treatment facilities needed Required via wastewater discharge permitting BOD5 of wastewater ranges 110 – 440 mg/L and by the end? Reduced by 95% The Wastewater Treatment Process Main goal is to remove and degrade organic matter under “controlled conditions” Three major steps: primary, secondary, tertiary treatment Primary Treatment Physical separation of large solids from the waste stream Metal grating  Bar screens  Grit tanks  primary settling tank ~50% of suspended organic solid settle as sludge in setting tank = primary sludge Secondary Treatment Biological degradation through decomposition by microorganisms Other benefit? Pathogens also reduced in number This biological treatment is performed by (1) aeration tanks (2) trickling filter beds (3) sewage lagoon Disinfection is usually done after one of these treatment steps Trickling Filters Composed of plastic units but older facilities composed of stones or plastic sheets Effluent pumped over onto a filter bed which allows for microbial growth and biofilms form Microorganisms in the biofilm intercept organics as it trickles past them and degraded or decomposes them aerobically Decomposition helps aid in biofilm maintenance as OM converts to microbial biomass via growth This biofilm is a zoogleal film composed of bacteria, fungi, algae, protozoa BOD removal by TFs? ~85% Schematic of biofilm in TF bed Vayenas 2011 Aeration tanks Activated sludge process where effluent mixed with bacterial-rich slurry Air or pure oxygen gas pumped into mixtures to encourage bacterial growth and aerobic decomposition Secondary sludge is removed, added to primary sludge, and anaerobically digested to produce biosolids Pathogens are removed during this activated sludge process from (1) other microbes outcompeting them in sludge (2) pathogen adsorption to sludge itself through physical processes Detention time in aeration tanks = 4-8 hours Activated Slduge Aeration tank contents Mixed liquor suspended solids (MLSS) is its contents Organic component of MLSS is mixed-liquor volatile suspended solids (MLVSS) Activated sludge must be maintained with a proper ratio of substrate (organic load) to microbes (food- to-microorganism ratio; F/M) F/M = (Q x BOD5)/(MLSS x V) where, Q = flow rate of sewage in million gallons per day (MGD) BOD5 we know! MLSS = in mg/L V = volume of aeration tank in gallons Aeration tank contents F/M is typically 0.2 to 0.5 lb BOD5/day/lb MLSS What this means: many more microbes than food Low F/M (given above) is what we want! Microbes that are starving will breakdown wastewater more efficiently F/M = (Q x BOD5)/(MLSS x V) where, Q = flow rate of sewage in million gallons per day (MGD) BOD5 we know! MLSS = in mg/L V = volume of aeration tank in gallons More about activated sludge Processes can be modified for nitrogen or phosphorous removal Nitrogen = encourage nitrification followed by denitrification >>>Nitrifying bacteria must be greater growth than other heterotrophic bacteria --- can be accomplished through an older sludge age as nitrifiers are slow growers >>>To encourage denitrification, anaerobic conditions must be set Phosphorous = modification of the A/O process >>> anaerobic/oxic process has an anaerobic zone added upstream of aeration tank Tertiary Treatment Further reduces organics, turbidity, N, P, metals, pathogens Physiochemical treatment such as…. Coagulation, Filtration, Activated Carbon Adsorption, Reverse Osmosis, Disinfection Pathogen Removal Processes before tertiary treatment remove most pathogens, but in order to reuse water we must make sure they are reduced even further!  Pathogen Removal Processes before tertiary treatment remove most pathogens, but in order to reuse water we must make sure they are reduced even further! Activated sludge removes 90% of enteric bacteria, up to 99% of enteroviruses and rotaviruses, up to 90% of Giardia and Cryptospiridium ”Crypto” is a genus of eukaryotic parasites that cause respiratory and GI illness ttps://www.youtube.com/watch?v=xHztEOsiZ9E Cryptospiridium Transmitted from animals to human via waste via zoonotic species Oocysts are excreted by host and then when in a new host, oocysts release sporozoites and then parasitize epithelial cells Oocysts are very hardy and difficult to disinfect! Tertiary treatment of a mixed-sand media filtration will remove almost all of these! Septic tanks Wastewater enters tank and oils rise to top, solids sink to bottom Anaerobic decomposition occurs that results in septic tank sludge Wastewater in tank for 24-72 hours before channeling to a drainage field but sometimes residual sludge must be pumped out. Contaminants are much greater in septic tanks and can pollute groundwater during leakage Beneficial for rural or suburban populations of low densities Wetlands https://texanbynature.org/projects/constructed-wetlands/ An additional treatment for secondary effluents via constructed wetlands Vegetation provides increased surface area for microbes and also helps filter and remove contaminants Two types of constructed wetlands: free water surface systems (FWS) and subsurface flow systems (SFS) FWS = natural wetland or marsh exposed to atmosphere SFS = channels filled with media to support vegetation Drinking Water Treatment In the past, very basic treatment processes were implemented such as slow sand filtration The germ theory of disease was created in 1870s, and Koch demonstrated chlorine kills bacteria in 1880s Disinfection used during typhoid fever caused large declines in outbreaks Process Train Simplest treatment - chlorination to disinfect Filtration may be added through sand or coal to reduce turbidity In-line filtration adds a coagulant to flocculate suspended particles for easier filtration Coagulation The addition of chemicals to remove dissolved and suspended solids by sedimentation and filtration Most common chemicals include hydrolyzing metal salts such “alum” or charged organic molecules called polyelectrolytes These can be added before or after flocculation but before the filtration step Flocculati on Physical process of stirring water to increase particle collision and promote larger particles coming together so filtration is easier Microorganisms in water will be trapped in these ”floccs” and removed Sedimentati Sedimentation allows settling of particles through a sand bed and filtration is filtering on & out particles Together, removal efficiencies will remove > Filtration 99.99% of microorganisms, including viruses  Filtration is an important barrier in the removal of protozoan parasites like Giardia lamblia and Filtration Cryptosporidium. Cysts and oocysts are resistant to inactivation by disinfectants so additional steps must be used to prevent illness Drinking Water Distribution Systems Once treated, water is transported miles to consumers During transport, quality can degrade due to several factors Reduced impact of disinfectant residual Biofilm sloughing Pipe breaks and pipe sediments Regrowth of bacteria in pipes themselves Microbial Growth Bacterial concentrations vary from < 1 CFU or as high as 106 CFU per milliliter in distribution water In pipe biofilms, up to 107 CFU per square centimeter Biofilms in pipe sections can be extremely variable and patchy --- low levels of organic matter will allow their growth Biofilms can coexist with chlorine residuals during distribution E. coli is 2400X more resistant when attached in a biofilm versus free-living cells Legionella in distribution systems More resistant to chlorine than even E. coli and small numbers can routinely survive in pipes Hot-water tanks in homes and hospitals favors growth Legionella survives well at 50C and can be capable of growth at 42C Organic Matter Growth is greatly influenced by biodegradable OM, temperature, detention time Example: Pseudomonas aeruginosa are able to grow in tap water with low concentrations of low-molecular weight organic substrates (acetate, lactate, amino acids) Amount of biodegradable OM is difficult to directly measure Assimilable organic carbon (AOC) in water is the biggest issue – 0.1-9% of total organic carbon can be AOC Monday Quiz 6 over todays lecture Go over Exam 2 Being Organic Pollutants chapter and last emerging topic