Wastewater treatment processes (I) ENV H 440/ENV H 541 John Scott Meschke Office: Suite 2249, 4225 Roosevelt Phone: Gwy-Am Shin Office: Suite 2339, 4225 Roosevelt Phone:

Key points Purpose of the individual unit processes The typical operating conditions The outcome of the processes Microbial reduction of the processes

How much wastewater do we produce each day? Wastewater Characteristics These values are rough estimates only and vary greatly by locale.

Wastewater treatment systems Decentralized –Septic tank –Waste stabilization ponds Facultative lagoon Maturation lagoon –Land treatment –Constructed wetland Centralized

Sewer systems

Typical composition of untreated domestic wastewater

Microorganism concentrations in untreated wastewater

(Minimum) Goals of wastewater treatment plants <30 mg/L of BOD 5 <30 mg/L of suspended solids <200 CFU/100mL of fecal coliforms

Conventional Community (Centralized) Sewage Treatment Pathogen Reductions Vary from: low ( %) Secondary Treatment Using Activated Sludge Process Sludge drying bed or mechanical dewatering process

Typical Municipal Wastewater Treatment System Preliminary or Pre- Treatment Primary Treatment Secondary Treatment Disinfection Sludge Treatment & Disposal

Preliminary Wastewater Treatment System Preliminary or Pre- Treatment Solids to Landfill

Preliminary Treatment Facilities Preliminary Treatment - Bar Racks Bar Racks: are used to remove large objects that could potentially damage downstream treatment/pumping facilities. Ref: Metcalf & Eddy, 1991

Preliminary Treatment - Grit chamber Grit chamber: used to remove small to medium sized, dense objects such as sand, broken glass, bone fragments, pebbles, etc.

Primary Wastewater Treatment Primary Treatment

Primary sedimentation To remove settleable solids from wastewater

Primary Clarification Primary Sludge Primary Effluent Influent from Preliminary Treatment Section through a Circular Primary Clarifier Primary Treatment Scum: Oil, Grease, Floatable Solids

Primary sedimentation To remove settleable solids from wastewater Average flow: gpd/ft 2 Retention time: hours (at maximum flow) % removal of suspended solids % removal of BOD 5 ~20 % removal of phosphate ~50 % removal of viruses, bacteria, and protozoa 90 % removal of helminth ova

Secondary Wastewater Treatment Secondary Treatment

Secondary treatment processes To remove suspended solids, nitrogen, and phosphate 90 % removal of SS and BOD 5 Various technologies –Activated sludge process –Tricking filter –Stabilization ponds

Secondary Treatment Using Activated Sludge Process Secondary Treatment Sludge drying bed or mechanical dewatering process

Aerobic microbes utilities carbon and other nutrients to form a healthy activated sludge (AS) biomass (floc) The biomass floc is allowed to settle out in the next reactor; some of the AS is recycled Secondary Treatment Simplified Activated Sludge Description

General Microbial Growth Carbon Source:Dissolved organic matter Energy Source:Dissolved organic matter Terminal Electron Acceptor: Oxygen Nutrients:Nitrogen, Phosphorus, Trace Metals Microorganisms:Indigenous in wastewater, recycled from secondary clarifier Secondary Treatment

Activated Sludge Aeration Basins Empty basin, air diffusers on bottom Same basin, in operation Secondary Treatment

The Oxidation Ditch Ref: Reynolds & Richards,1996, Unit Operations and Processes in Environmental Engineering Secondary Treatment

The Oxidation Ditch Ref: Reynolds & Richards,1996, Unit Operations and Processes in Environmental Engineering Secondary Treatment

Circular Secondary Clarifier Secondary Effluent Influent from Activated Sludge Aeration Basin or Trickling Filter Section through a Circular Secondary Clarifier Return (Secondary) Sludge Line Secondary Treatment

Activated sludge process To remove suspended solids, nitrogen, and phosphate Food to microorganism ratio (F:M ratio): 0.25 kg BOD 5 per kg MLSS (mixed liquor suspended solids) per day at 10 o C or 0.4 kg BOD 5 per kg MLSS per day at 20 o C Residence time: 2 days for high F:M ratio, 10 days or more for low F:M ratio Optimum nutrient ratio: BOD 5 :N:P =>100:5:1 90 % removal of SS and BOD 5 ~20 % removal of phosphate > 90 % removal of viruses and protozoa and % removal of bacteria

Secondary Treatment Using Trickling Filter Process Secondary Treatment Trickling Filter

Primary effluent drips onto rock or man-made media Rotating arm to distribute water evenly over filter Rock-bed with slimy (biofilm) bacterial growth Primary effluent pumped in Treated waste to secondary clarifier

Trickling Filter

Tricking filter process To remove suspended solids, nitrogen, and phosphate Organic loading (BOD 5 X flow/volume of filter): 0.1 kg BOD 5 per m 3 per day Hydraulic loading: 0.4 m 3 per day per m 3 of plan area 90 % removal of SS and BOD 5 ~20 % removal of phosphate Variable removal levels of viruses, % removal of bacteria and > 90 % removal of protozoa

Stabilization Ponds The oldest wastewater treatment systems – Requires a minimum of technology – Relatively low in cost – Popular in developing countries and small communities in the US (90 % communities with population <10,000) Used for raw sewage as well as primary ‑ or secondary ‑ treated effluents. Facultative ponds and aerated lagoons Ponds and Lagoons

Facultative Pond Ponds and Lagoons

Facultative ponds 3 zones: upper photic (aerobic) zone, facultative (aerobic and anaerobic) zone and lower anaerobic zone. –Upper aerobic zone: algae use CO 2, sunlight and inorganic nutrients (photosynthesis) to produce oxygen and algal biomass. –Facultative zone: bacteria and other heterotrophs convert organic matter to carbon dioxide, inorganic nutrients, water and microbial biomass. –Lower anaerobic zone: anaerobic bacteria degrade the biomass from upper zones Influence by many factors –Sunlight –Temperature –pH –Biological activities –Characteristics of wastewater Ponds and Lagoons

Facultative ponds To remove suspended solids, nitrogen, phosphate, and pathogens Operating water depth: meters (maximum) BOD loading: g/m 3 /day Retention time: 3-6 months >90 % SS and BOD removal (warm and sunny climates) Microbe removal may be quite variable depending upon pond design, operating conditions and climate. –90-99% removal of indicator and pathogenic bacteria –99 % removal of PV1 –99.9 reduction of Giardia and Cryptosporidium

Aerated Lagoons Ponds and Lagoons Stabilization Lagoon Aerated Lagoons

Aerated lagoons Biological activity is provided by mainly aerobic bacteria Influence by many factors –Aeration time –Temperature –pH –Biological activity –Characteristics of wastewater

Aerated lagoons To remove suspended solids, nitrogen, phosphate, and pathogens Operating water depth: 1-2 meters Retention time: <10 days 85% BOD removal (at 20 o C and an aeration period of 5 days) 65% BOD removal (at 10 o C and an aeration period of 5 days) Microbe removal may be quite variable depending upon pond design, operating conditions and climate

Wastewater Disinfection Disinfection

Typical Municipal Wastewater Treatment System Preliminary or Pre- Treatment Primary Treatment Secondary Treatment Disinfection Sludge Treatment & Disposal

Sludge processing Thickening Digestion Dewatering Disposal

Sludge thickening To reduce the volume of sludge –to increase sludge solids at least 4 % Gravity thickening and mechanical thickening Gravity thickening –Used for primary and tricking filter solids –Without chemical flocculants –loading rate: kg/m 2 per day Mechanical thickening –Used for activated sludge solids –With chemical flocculants –dissolved air flotation, gravity belt thickeners, and centrifuge thickening – loading rate: kg/m 2 per day (dissolved air flotation), L/m (gravity belt thickeners), L/m (centrifuge thickening) The concentration of pathogens increased during this process

Gravity belt thickener

Regulatory requirement for disposal of sewage sludge Class B biosolids (agriculture land) –< 2 million MPN/g of fecal coliforms –Seven samples over 2-weeks period – ~2 log removal Class A biosolids (home lawn and garden) –< 1000 MPN/g of fecal coliforms –< 3 MPN/4g of Salmonella sp. –< 1 PFU/4g of enteric viruses –< 1/4g of Helminth ova –~ 5 log removal

Processes to significantly reduce pathogens (PSRP) for a Class B biosolids Aerobic digestion Anaerobic digestion Air drying Composting Lime stabilization

Digestion To stabilize organic matter, control orders, and destroy pathogens Aerobic digestion and anaerobic digestion Aerobic digestion –Sludge is agitated with air/oxygen –loading rate (maximum): 640 g/m 2 per day –Temperature and retention time: 68 o F for 40 days or 58 o F for 60 days –Solids and BOD reduction: %

Anaerobic digestion Sludge is treated in the absence of air Operation conditions (optimum) –Temperature: o F (98 o F) –pH: ( ) –Alkalinity: mg/L –Solid loading: lb/ft 3 /day –Retention time: days Treatment outcome –Solid reduction: % –Significant reduction of most pathogens –Gas production: methane and carbon dioxide

Anaerobic digester

Air drying, composting, and lime stabilization Air drying –Sludge is dried on sand beds/(un)paved basins –Retention time: minimum of 3 months Composting –Various methods: in-vessel, static aerated file, and periodically mixed windrows –File temperature should be raised > 40 o C for 5 days –For 4 hours during the 5 days, the file temperature should be > 55 o C Lime stabilization –Sufficient lime should be added to raise the pH 12 after 2 hour contact –4 log inactivation of enteric viruses, 2-7 log inactivation of indicator bacteria, no inactivation of Acaris ova

Processes to further reduce pathogens (PFRP) for a Class A biosolids Heat drying –Sludge is dried by contact with hot gases –The temperature of gas is >80 o C Thermophilic aerobic digestion –Sludge is agitated with air/oxygen – o F for 4-20 hours Pasteurization –158 o F for 30 minutes Beta- or gamma ray irradiation –Sludge is irradiated with either beta- or gamma ray –> 1.0 Mrad at room temperature

Dewatering To concentrate sludge by removing water Pressure filtration, centrifugation, and screw press Pressure filtration (belt filter press and plate- and-frame filter) –Usually with polymer flocculation –Loading rate: gpm/m (hydraulic) and lb/m/h (solid) –Feed solid: 1-6 % –Cake solids: %

Belt filter press

Disposal Land application Landfill Incineration Ocean dumping (no longer allowed in US)