Biological Wastewater Treatment

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this book describes the methods for bilogical treatment of wastewater.

Text of Biological Wastewater Treatment


UNIT IIWASTEWATER BIOTREATMENT Biological treatment of waste waterMethodsAerobic : removal of organic pollutants in wastewater by bacteria that require oxygen to work End products: Water and carbon dioxide and biomassAnaerobic :bacteria digests biosolids in the absence of oxygen End products: methane and carbon dioxide gas and biomass

Aerobic Biological TreatmentSteps Primary secondaryTertiary treatments

Aerobic Biological Treatment TechnologiesConventional Activated Sludge Process (CASP) SystemCyclic Activated Sludge System (CASS)Membrane Bioreactor (MBR)Biofilm reactors in aerobic biological wastewater treatment: - Trickling Filters - Integrated Fixed Film Activated Sludge (IFAS) System - Submerged Aerobic Fixed Film Reactor - Fluidized Bed Bioreactor - Hybrid Biofilm/suspended Growth processes - Rotating biological contactors

Conventional Activated Sludge Process (CASP) Systemmost common and oldest biotreatment process

used to treat municipal and industrial wastewater

wastewater after primary treatment i.e. suspended impurities removal is treated in an activated sludge process based biological treatment system comprising aeration tank followed by secondary clarifier.

The aeration tank is a completely mixed or a plug flow (in some cases) bioreactor where specific concentration of biomass (wide variety of microorganisms) (measured as mixed liquor suspended solids (MLSS) is maintained along with sufficient dissolved oxygen (DO) concentration (typically 2 mg/l)

biodegradation of soluble organic pollutants (measured as Chemical oxygen demand (COD) and Biological oxygen demand (BOD)The aeration tank is provided with fine bubble diffused aeration pipework at the bottom to transfer required oxygen to the biomass and also ensure completely mixed reactor.The aerated mixed liquor from the aeration tank overflows to the secondary clarifier unit to separate out the biomass and allow clarified, treated water to the downstream filtration system for finer removal of suspended solids. The separated biomass is returned to the aeration Tank by means of return activated sludge (RAS) pump.

Activated sludge plantCyclic Activated Sludge System (CASS)Cyclic Activated Sludge System (CASS) - one of the most popular sequencing batch reactor (SBR) processes employed to treat municipal wastewater and wastewater from a variety of industries including refineries and petrochemical plants. This technology offers several operational and performance advantages over the conventional activated sludge process. The CASS SBR process performs all the functions of a conventional activated sludge plant (biological removal of pollutants, solids/liquid separation and treated effluent removal)

by using a single variable volume basin in an alternating mode of operation, thereby dispensing with the need for final clarifiers and high return activated sludge pumping capacity. The Cyclic Activated Sludge System (CASS), incorporates a high level of process sophistication in a configuration which is cost and space effective and

offers a methodology that has operational simplicity, flexibility and reliability that is not available in conventionally configured activated sludge systems. The reactor basin is divided by baffle walls into three sections Zone 1: Selector, Zone 2: Secondary Aeration, Zone 3: Main Aeration). Sludge biomass is intermittently recycled from Zone 3 to the Zone 1 to remove the readily degradable soluble substrate and favor the growth of the floc-forming microorganisms. System design is such that the sludge return rate causes an approximate daily cycling of biomass in the main aeration zone through the selector zone. No special mixing equipment or formal anoxic mixing sequences are required to meet the effluent discharge objectives.

CASS utilizes a simple repeated time-based sequence which incorporates: 1 & 2) Aeration (for biological reactions) 2) Settle (for solids-liquid separation) 3) Decant (to remove treated effluent)

Characteristics of aerobic activated sludge:Overall, activated sludge must contain a microorganisms capable of producing all enzyme systems required for the biodegradation of both soluble and insoluble pollutants.Activated sludge contains prokaryotes (bacteria) and Eukaryotes (Protozoa and fungi)The primary consumers of organic wastes are the heterotrophic (An organism that cannot synthesize its own food and is dependent on complex organic substances for nutrition) bacteriaThe majority of the bacterial genera in activated sludge are Gram-negativePseudomonas, Arthrobacter, Comamonas, Lophomonas, Zoogloea, Sphaerotilus, Azotobacter, Chromobacterium, Achromobacter,Flavobacterm, Bacillus, and Nocardia.Certain genera Sphaerotilus or Nocardia-causes poor settlingMany types of protozoa : 50,000 cells/ml

Design of activated sludge processThe main parameters that have to be taken into account when designing activated sludge systems are:

Influent : Characteristics must be known before treatment- Flow rate of the inflow of waste water- Substrate concentration in inflow of waste water (For e.g. BOD)- Active biomass concentration in inflow of wastewater

Kinetic data and stoichiometric coefficientsSludge recycle rate Sludge wasting rateOxygen uptake rate

hydraulic loading rate, HLR kg BOD/m3/day = raw BOD (kg/m3) x flow rate (m3/day) aeration volume (m3) hydraulic retention time (HRT, h) = reactor volume (m3) flow (m3/h)

sludge loading rate SLR, kg BOD/kg MLSS/day = raw BOD (kg/m3) x flow rate (m3/day) MLSS (kg/m3) x aeration tank volume (m3)Membrane Bioreactor (MBR) Latest technology for biological degradation of soluble organic impurities.

Extensively used for treatment of domestic sewage but for industrial waste treatment applications, its use has been somewhat limited or selective.

MBR is an improvement of CASP, where the secondary clarifier is replaced by a membrane unit for the separation of treated water from the mixed solution in the bioreactor

System comprises of activated sludge process with the biomass separation carried out by membrane processSeparation of biomass using membrane provides filtered quality final effluent can reuse

Biofilm reactors in aerobic biological wastewater treatment2 categories: Fixed-medium systems : biofilm media are static in the reactors and the biological reactions take place in the biofilm developed on the static media, and Moving-medium systems : biofilm media are kept continually moving by means of mechanical, hydraulic, or air forces. Biofilms can be used in various types of reactors Trickling filters and biological towersRotating biological contactorsFed Batch Reactors (FBRs)Granular media filtersFluidized bed biofilm reactorsHybrid biofilm/suspended growth processes


Flow Diagram for Trickling Filters

Trickling FiltersNot a true filtering or sieving processMaterial only provides surface on which bacteria to growCan use plastic medialighter - can get deeper beds (up to 12 m)reduced space requirementlarger surface area for growthgreater void ratios (better air flow)less prone to plugging by accumulating slimeTrickling Filters

Filter MaterialTypical Trickling Filter

Random Packing

Structured MediaBio-towers

Trickling FilterTank is filled with solid media Rocks Plastic Bacteria grow on surface of media Wastewater is trickled over media, at top of tank As water trickles through media, bacteria degrade BOD Filter is open to atmosphere, air flows naturally through media Treated water leaves bottom of tank, flows into secondary clarifier Bacterial cells settle, removed from clarifier as sludge Some water is recycled to the filter, to maintain moist conditions

29Trickling Filter System

30Trickling Filter Process

31Types of Trickling FiltersStandard or low ratesingle stage rock media unitsloading rates of 1-4 m3 wastewater/m2 filter cross-sectional area-daylarge area requiredHigh ratesingle stage or two-stage rock media unitsloading rates of 10-40 m3 wastewater/m2 filter cross-sectional area-dayre-circulation ratio 1-3Super ratesynthetic plastic media units- Plastic media depths of 5-10 mmodules or random packedspecific surface areas 2-5 times greater than rockmuch lighter than rockscan be stacked higher than rocksLoading rates of 40-200 m3 wastewater/m2 filter cross-sectional area-day

Design Criteria for Trickling Filters

Submerged Aerobic Fixed Film Reactor

cost-effective method of waste water treatment

sewage sanitation that is primarily used in residential and commercial complexes

This equipment primarily works on the three stages that are Primary Settlement, Secondary Treatment and Final Settlement / Clarification

Improved treatment quality

particularly for small to medium sized treatment plants where available land is limited, and where full time operational manning would be uneconomical

A well built Submerged Aerated Filter plant has no moving parts within its main process zones, any serviceable items will be positioned to access easily without disrupting the ongoing sewage treatment

Fluidized Bed Bioreactor

Fluidized bed bioreactors (FBR) have been receiving considerable interest in wastewater Treatment.

A fluidized bed bioreactor consists of microorganism coated particles suspended in wastewater which is sufficiently aerated to keep the gas, liquid and the solid particles thoroughly mixed.

capable of achieving treatment in low