The Welsh Area would like to welcome you to today’s webinar The Science Behind Wastewater Treatment Joshua Williams Process and Commissioning Scientist Welsh Water joshua.williams@dwrcymru.com For sponsorship opportunities contact: Record your CPD online by using the IW Welsharea.instituteofwater@outlook.com exclusive online portal. Go to ‘My Dashboard’ on your membership home For membership information contact: screen and select ‘My CPD’ Gemma.Williams@dwrcymru.com
The ancient history of wastewater treatment • Sanitation has played a pivotal part in human development, allowing for rapid growth and improvements to life expectancy. • Forms of wastewater management have existed since the start of human development • Some examples from our ancient history • Domestic wastewater for irrigation – Bronze Age (ca. 3200-1100 BCE). • The Mesopotamians introduced the world to clay sewer pipes around 4000 BCE • The ancient Greeks had a well-organized water system for taking out waste water and storm sewage canals for overflow when there was heavy rain. • In medieval European cities, small natural waterways used for carrying off wastewater were eventually covered over and functioned as sewers. The majority was discharged direct to watercourses.
The modern history of wastewater treatment • The tremendous growth of cities during the Industrial Revolution quickly led to terribly over polluted streets, which acted as a constant source for the outbreak of disease. • As part of a trend of municipal sanitation programs in the late 19th and 20th centuries, many cities constructed extensive sewer systems to help control outbreaks of disease such as typhoid and cholera. • Initially these systems discharged sewage directly to surface waters without treatment. Later, cities attempted to treat the sewage before discharge in order to prevent water pollution and waterborne diseases. • Early techniques involved land application of sewage on agricultural land. • In the late 19th century some cities began to add chemical treatment and sedimentation systems to their sewers. Most cities in the Western world added more expensive systems for sewage treatment • in the 20th century, after scientists at the University of Manchester discovered the sewage treatment process of activated sludge in 1912. • Some of the biggest developments took place in major cities across the United Kingdom, this included Manchester, Liverpool and London.
The wastewater treatment process • Before waste water can be treated it needs to be collected. Every day in the UK over 624,200 kilometers of sewers collect over 11 billion liters of wastewater from homes, municipal, commercial and industrial premises and rainwater run-off from roads and other impermeable surfaces. • The treatment provided at waste water treatment plants can involve: • Preliminary treatment – to remove grit and gravel and screening of large solids. • Primary treatment – to settle larger suspended, generally organic, matter. • Secondary treatment – to biologically break down and reduce residual organic matter. • Tertiary treatment – to address different pollutants using different treatment processes. • This is treated at about 9,000 sewage treatment works before the treated effluent is discharged to inland waters, estuaries and the sea.
• Primary purpose of primary treatment is to reduce solids and Primary Treatment organic matter loadings.
Secondary Treatment – Biological Filters • One of the earlier methods of treating sewage was to run it into a tank filled with loose stone. These units were known as contact filters. • A modification to convert them to continuous operation, rather than batch, was made in 1893. • Biological Principles • This is an aerobic process • Sewage is a suitable source of food for the micro-organisms in a biological filter as it contains nitrogen, organic carbon compounds, phosphorus and trace elements. • Air circulates in the voids between the media, taking oxygen to the slime layer on the surface of the media. • As sewage trickles over the media, the various organic substances are absorbed onto the biological film thus supplying the organisms with food. • If either food or oxygen is absent, this metabolism will stop. • This process is most efficient when the slime layer is thin and totally aerobic.
Secondary Treatment – Biological Filters • The microorganisms absorb the organic matter in the sewage and stabilize it by aerobic metabolism, thereby removing oxygen-demanding substances from the sewage. Trickling filters remove up to 85 percent of organic pollutant from sewage. • Filters require the correct loading to operate efficiently this is either achieved via the PSTs or in a side stream process for example a High Rate Filter (HRF) • Overloading can result in poor Ammonia removal, • Anaerobic decomposition, excess biomass • Hydraulic loading also plays a major part in the health of a biological filter • Wetting rates – max 0.75 m3/m2/d. • Sk factors – 5 mm/pass
Microorganisms Flagellates Nitrosomonas Europaea • Bacteria play the major part in the treatment process • Nitrification takes place through the presence of Nitrifying bacteria Nitrosomonas & Nitrobacter • Typically these are too small to see under the microscope so a range of other microorganisms are used to determine the health and condition of the biomass • Protozoa : Feed on biofilm bacteria and helps in maintaining high decomposition rate, examples : • Flagellates (e.g., Bodo, Monas) • Ciliates (e.g., Vorticella) Rotifers • Amoeba (e.g., Amoeba, Arcella) • Rotifers • Macroinvertebrates : Ciliates (Vorticella) • Diptera (Fly) • Nematodes (hookworm, lungworm etc.) Nematodes
• The activated sludge process is a method of treating sewage in an aerated bacterial sludge. effluent by settlement. • Most of the settled activated sludge is returned for re- use referred to as Activated Sludge returned activated sludge (RAS) • The excess is discharged as surplus activated sludge. (SAS). • Treatment is optimised by maintaining the correct food to mass ratio (F:M) and dissolved oxygen. • Microscopic analysis aids the ability to maintain a stable process • Changes to protozoa populations can indicate issues such as • Oxygen deficiency • Shock loading • Toxicity • The objective is to achieve a ideal F:M ration and sludge age
Activated Sludge – Filamentous Bacteria • Filamentous bacteria are present in some abundance in all activated sludge and some aid the flock formation process • Low dissolved oxygen (DO), nutrient deficiency, In appropriate F/M, and septicity can cause a rapid growth in certain types of filamentous bacteria • This can have detrimental effects on the treatment process resulting in poor solids settlement Ideal sludge : Balance between filaments and floc-forming organisms. Bulking : Excessive quantities of filamentous organisms observed. Weak flocs : Absence of filamentous organisms.
Humus/Final Settlement • Same principle and primary settlement tank • Retention time should be 2 hours • Typically radial flow circular tanks • Hydraulics important to aid settlement • Humus sludge shall generally be returned to primary settlement tanks for co-settlement
Chemical Phosphorus Chemical addition Removal • The objective is to remove phosphorus from the final effluent of treatment works to improve the water quality of rivers • Chemical removal is the most common form of treatment used in the industry, however biological treatment and solids removal technologies exist. • The objective of chemical removal is to flocculate the phosphorus from the soluble form to a solid form using a metal salt. • The solids are settled out using a sites primary settlement tanks or humus settlement tanks and processed as part of the sludge stream • The most common metal salts are Iron and Aluminum based • The main issues are around alkalinity effects and the non selective nature of these chemicals
Chemical Phosphorus Removal • Molar ratios vary depending on target results and a number of factors that can influence them are; • pH • Mixing method • Wastewater characteristics • Colloids and solids effect P-metal hydroxide complexations • Organic substrates • Iron and aluminum can react with humic substances • The correct dose rates to achieve this are done on a site to site basis with sampling and jar test results (examples shown later)
Emerging Technologies • In recent years a lot of focus has switched to several emerging pollutants • These include pharmaceuticals and more recently microplastics • As the industry comes to terms with the possibility of having to treat these in the future, research and development is underway to review how we can meet them. • However although the basics have worked well for 100s of years, research continues everyday to look at different more energy efficient ways of treating wastewater • A few examples we will look at today are; • Membrane Biofilm Reactor (MBfR) • Nanoparticle Treatment • Passive aeration
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