Domestic Waste Water Treatment

Introduction

• Modern sewage treatment began in the early 20th century
• Initial processes focused on treating organic matter in domestic waste.
• Current treatment priorities include reduction of pathogens and removal of toxic substances, driven by water scarcity and reuse needs.

Domestic Waste Water

• Consists mainly of human waste (feces, urine) and "gray water"
• Gray water comes from washing, bathing, and meal preparation.
• Industrial and commercial water could also be part of the system
• Humans excrete approximately 100-500 grams of feces and 1-1.3 liters of urine per day, per person.

Composition of Untreated Domestic Water

• Solids range: 350 (low) to 1200 (high) mg/L.
• Dissolved solids range: 250 (low) to 850 (high) mg/L.
• Volatile solids range: 105 (low) to 325 (high) mg/L.
• Suspended solids range: 100 (low) to 350 (high) mg/L
• Volatile suspended solids range: 80 (low) to 275 (high) mg/L.
• Settleable solids range: 5 (low) to 20 (high) mg/L.
• Biological Oxygen Demand ranges: 110 (low) to 400 (high) mg/L.
• Total Organic Carbon ranges: 80 (low) to 290 (high) mg/L.
• Chemical Oxygen Demand ranges: 250 (low) to 1000 (high) mg/L.
• Total Nitrogen: 20 (low) to 85 (high) mg/L.
• Phosphorus: 4 (low) to 15 (high) mg/L.

Microorganisms in Untreated Domestic Water

• Includes total coliform, fecal coliform and streptococci, enterococci, Shigella, and Salmonella
• Also contains Clostridium perfringes, Giardia and Cryptosporidium cysts, helminth ova, and enteric viruses.

Sewage Composition

• Sewage is primarily water (99.9%) and solids (0.1%).
• Solids are 70% organic and 30% inorganic.
• Organic portion: 65% nitrogenous, 35% non-nitrogenous.
• Nitrogenous organics contain proteins.
• Non-nitrogenous organics consist of carbohydrates (25%) and lipids (10%).
• Inorganic portion consists of salts, grits, and metals

Tests for Organic Matter

• Total Organic Carbon (TOC): Determined by oxidation of organic matter followed by CO2 measurement using infrared analysis.
• Biochemical Oxygen Demand (BOD): Measures dissolved oxygen consumed by microorganisms during organic matter oxidation, including carbonaceous and inorganic matter.
• Chemical Oxygen Demand (COD): Measures oxygen required to oxidize all organic carbon to CO2 and H2O.

Objectives of Sewage Treatment

• Convert waste into a reusable resource
• Prevent water body pollution
• Lower BOD from 30 mg/l to approximately 20 mg/l in the final effluent.
• Eliminate waterborne disease pathogens

Modern Wastewater Treatment Steps

• Primary Treatment: Physical process separating debris followed by sedimentation
• Secondary Treatment: Biological breakdown by microorganisms.
• Tertiary Treatment: Physico-chemical process to remove turbidity

Secondary Treatment / Biological Treatment

• Involves Aerobic and Anaerobic processes
• Two types include Attached Growth Treatment and Suspended Growth Treatment
• Attached Growth Treatment includes Trickling Filters and Rotating Biological Contactors
• Suspended Growth Treatment involves Activated Sludge process

Trickling Filters

• These are an older form of aerobic oxidation treatment and can also be known as bacteria beds or percolating filters.
• Media size is 50-100cm and 1.5 to 2.0 meter depth
• Rotating arm sprays sewage, saturating liquid with oxygen
• Microorganisms create a zoogloeal film
• Biofilm thickness limits oxygen diffusion causes film sloughing, leading to new biofilm development.
• Humus tank collects humus sludge.
• Oxidation rate depends on oxygen availability
• Also used in treating industrial waters

Trickling Filter Microorganisms

• Upper level includes photosyntheti algae like Phormidium, Chlorella, and Ulothrix
• Zoogloeal film contains Sphaerotilus natans and Beggiatoa
• Bacteria include: Pseudomonas, Flavobacterium, Achromobacter and Alcaligens
• Lower level includes Nitrobacter and Nitrosomonas
• Various fungi and protozoa are present

Trickling Filter: Types based on Hydraulic and Organic Loading Rates

• Low rate: Filters used for domestic sewage
• High and Super rate: Filters used for Industrial sewage

Biotowers

• Modern trickling filters with plastic media like PVC or polypropylene are utilized
• Lightweight plastic allows stacking as biotowers, reaching 6-10 meters high.

Trickling Filters-Performance Factors

• Media type
• Organic loading
• Recirculation rate
• Flow distribution

Trickling Filters - Advantages

• Simple
• Requires less space
• Has low operating cost
• Efficient in hot climates

Trickling Filters - Disadvantages

• Moderate BOD removal at 75%
• Requires proper disposal of excess sludge
• Requires primary sedimentation

Rotating Biological Contactors (RBC)

• Modern device that operates on aerobic attached growth on moving media
• Used for domestic and industrial sewage treatment where pulp and meat etc are present
• Biofilm grows on bio-discs submerged in the sewage, then oxidize substances
• Efficient for 90% removal of organic matter

Rotating Biological Contactors Composition

• Composed of space and light circular discs made of polystyrene, PVC or polyethylene
• Discs assembled on a horizontal shaft and submerged in the path of waste water
• The shaft is rotated slowly at less than 10 revolution or per minute
• Discs are 40 - 60% submerged in sewage Rotating shaft will pick a thin layer of sewage to oxidize absorbed substances

Factors affecting RBC performance

• Rotation speed of the shaft
• Wastewater retention time
• Temperature
• Disc submergence
• Organic loading
• Media density

RBC - Advantages

• Compact and requires moderate energy input
• High BOD removal efficiency

RBC - Disadvantages

• Sludge disposal is a major issue
• Can often produce unpleasant odor

Activated Sludge Process (ASP)

• Effluent from primary treatment mixed with bacteria in a tank
• Air or oxygen promotes bacterial growth and decomposition
• Treated water goes to settling tank where water is siphoned off and sludge is removed
• Some sludge recycled as inoculum, remainder removed for disposal.
• Pathogen reduction via antagonistic microorganisms and adsorption.

Activated Sludge Process - Detention time & Composition

• Sewage detention time is 4 to 8 hours
• Tank content referred to as mixed liquor suspended solids (MLSS)
• The MLSS is organic and called MLVSS (mixed liquor volatile suspended solids)
• The MLVSS can contain non-microbial, dead, or living matter

Activated Sludge Process - Ratio & Measurements

• Should maintain proper food - to - microorganisms (F/M) ratio to remain proper
• F/M = (Q x BOD) / (MLSS / V)
• Q = sewage flow rate in million gallons a day
• BOD = biochemical oxygen demand (mg/L)
• MLSS = mixed liquor suspended solids (mg/L)
• V = aeration tank volume

Activated Sludge Process- Important Parameters

• Organic loading rates
• Oxygen supply
• Control and operation of settling tank
• Sludge settleability determined by sludge volume index (SVI)
• SVI = (V x 1000) / MLSS where V = settled sludge volume after 30 min (ml/L)

Floc

• Biomass in activated sludge exists as suspended flocs.
• Floc is activity unit in activated sludge
• A floc is an agglomeration of several millions of bacteria cells embedded in an extra matrix of fungi / protozoa
• Floc size ranges from 0.2 to 1mm in diameter

Floc - ASP Operation

• Successful operation of ASP depends on: ability to absorb substrates, assimilating and oxidizing organic material, oxidizing nitrogen
• Also requires efficient sedimentation of sludge
• Flocculation affected by growth rate, population composition, substrate concentration and nutrient balance

Activated Sludge - Advantages & Disadvantages

• Require less space to operate with high final effluent quality
• BOD value is higher at one end while bacteria will be more active

Microorganisms in ASP

• Zooglea ramigera is main organism for flocculation in ASP by extracellular slime matrix.
• Bacteria involved: Pseudomonas, Nitrosomonas, Flavobacterium, Alcaligenes, etc.
• Minor genera: Aeromonas, Aerobacter, Micrococcus, Spirillum, etc.
• Fungi: Zoophagus, Arthrobotrys, Geotrichum, etc.
• Protozoa: Opercularia, Vorticella, Aspidisca, and Chilodonella

Problems with Flocculation

• Bulking: Filamentous bulking resulting from organisms or Non-Filamentous bulking due to the excessive hydration of floc matter can occur
• Rising Sludge: Anoxic zone results from released nitrogen leading to evolution of the nitrogen gas- increases the buoyancy