MIC 303 INDUSTRIAL AND ENVIRONMENTAL MICROBIOLOGY CHAPTER 10-SEWAGE (WASTEWATER) TREATMENT

Composition of Domestic Wastewater Combination of human and animal excreta (feces and urine) and gray water resulting from washing, bathing and cooking. Mainly composed of: Proteins (40-60%) Carbohydrates (25-50%) Fats and oils (10%) Urea derived from urine Trace organic compound (inc pesticides, surfactants, phenols and pollutants such as non metal (As, Se), metals (Cd, Hg, Pb), benzene compounds, chlorinated compounds.

Overview of Wastewater Treatment Treatment methods based on chemical and biological processes called unit processes. ↓ Chemical unit processes include disinfection, adsorption or precipitation. Biological unit processes involve microbial activity, responsible for organic matter degradation and removal of nutrients.

Wastewater Treatment Processes Wastewater treatment comprises four steps: 1) Preliminary treatment : to remove debris and coarse materials that may clog equipment in the plant. 2) Primary treatment : treatment by physical processes such as screening and sedimentation. 3) Secondary treatment : Nutrient removal also generally occurs during secondary treatment of wastewater. Biological (eg: activated sludge, trickling filter, oxidation ponds) Chemical (e.g. disinfection) 4) Tertiary or advanced treatment : Biological and chemical unit processes used to further remove BOD, nutrients, pathogens and parasites and toxic substances.

Wastewater Treatment Processes (Con’t) Primary treatment Removal of solids Disinfection Secondary treatment Removal of much of the BOD Disinfection Water can be used for irrigation Tertiary treatment Removal of remaining BOD, N, and P Disinfection Water is drinkable

Municipal Sewage Treatment

PRIMARY TREATMENT-Septic Tanks Often use to discard domestic human waste for homes and business in areas of low population density that are not connected to municipal sewage systems. Septic tanks: a device whose operation is similar in principle to primary treatment. Sewage enters a holding tank and suspended solids settle out. The sludge in the tank must be pumped out periodically and disposed off. The effluent flows through a system of perforated piping into a leaching (soil drainage) field. The effluent enter the soil → decomposed by soil microorganisms.

Septic Tanks (Con’t) Limitations: The system work well when not overloaded and drainage system is properly sized to the load and soil type. For heavy clay soils, require extensive drainage systems → soil’s poor permeability. High porosity of sandy soils can results in chemical or bacterial pollution of nearby water supplies.

Primary Treatment: Septic Tanks

Oxidation Ponds Use by small communities and industries. Sewage (livestock waste) is dump into ponds called lagoons or stabilization ponds. Inexpensive but require large areas of land. Incorporate 2 stages: 1)Pond 1: Settle solids, pump water to pond 2 (analogous to primary treatment). The effluent will be pumped into second pond. 2)Pond 2: Pond is aerated by wave action. Difficult to maintain aerobic condition for bacterial growth. However, growth of algae helps to use carbon dioxide, thus producing oxygen for bacterial consumption. Bacterial decomposition of dissolved organic matter in water.

Oxidation Ponds

SECONDARY SEWAGE TREATMENT Predominantly biological: 1)Reduce most of remain organic matter from primary treatment. 2)Reduce the BOD. Sewage undergoes strong aeration → to encaurage the growth of aerobic bacteria and other microorganisms that oxidize the dissolved organic matter to carbon dioxide and water. Two common methods: 1)Activated sludge systems 2)Trickling filters

Activated sludge systems Use aeration tanks: air or pure oxygen is passed through the effluent from primary treatment. Inoculum (Effluent from primary treatment) also termed “activated sludge”. Activated sludge contains large numbers of sewage- metabolizing microbes, esp species of Zoogloea bacteria. This microbial community form bacteria-containing masses in the aeration tanks called floc (sludge granules) The soluble organic matter in the sewage is incorporated into the floc and its microorganisms. Aeration is discontinued after 4 to 8 hours and the contents of tank are transferred to a settling tank, the floc settles out → removing much of the organic matter. The remove solids are treated in anaerobic sludge digester (sludge didestion).

Activated sludge systems The clear effluent is disinfected and discharged (Tertiary Treatment). Activated sludge systems quite efficient → remove % of the BOD from sewage. Limitations: Phenomena of “bulking” (the sludge will float rather than settle out. The organic matter in the floc will flows out with the discharge effluent → resulting in local pollution. Cause by filamentous bacteria (e.g Sphaerolitus natans and Nocardia sp).

An Activated Sludge System

Trickling Filters The sewage is sprayed over a bed of rocks or molded plastic. A biofilm of aerobic microbes grows on the rock or plastic surfaces. Air circulates throughout the rock bed, microorganisms attached in the bioflim will oxidize organic matter trickling over the surfaces into carbon dioxide and water. Less efficient than activated sludge system, remove % of BOD. Adv: less troblesome to operate and have fewer problems from overloads or toxic sewage.

A Trickling Filter

Sludge Characteristics Two classes of sludge: Class A sludge: contains no detectable pathogens. Class B sludge: treated to reduce numbers of pathogens below certain levels. Can be used as fertilizers (biofertilizer) Disadv: potential problem is contamination with heavy metals that are toxic to plants.

Sludge Digestion The sludge resulted in secondary treatment often pumped to anaerobic sludge digesters. Carried out in large tanks and oxygen is almost excluded. Anaerobic sludge digester designed to encourage the growth of anaerobic bacteria (esp methane-producing bacteria) that degrade organic solids to soluble substances and gases, mostly methane (60-70 %) and CO 2 (20-30 %). Methane used as a fuel for heating the digester and frequently used to run power equipment in the plant. This system resulting in a large amounts of undigested sludge, but it is stable and inert. This sludge is pumped to shallow drying beds or water- extracting filters. Then, it will used for landfill or as a soil conditioner (termed as biosolids).

Figure Anaerobic Sludge Digester

TERTIARY SEWAGE TREATMENT Secondary effluent contains Residual BOD 50% of the original nitrogen 70% of the original phosphorus Tertiary treatment relies on physical and chemical methods, removes these by: Filtration through sand and activated charcoal → remove small particulate matter and dissolved chemicals. Chemical precipitation → Lime, alum and ferric chloride precipitate phosphate compounds. Nitrogen is converted to ammonia and discharged into the air in stripping towers. The purified water is the clarified and safe for drinking.