Wastewater Treatment Technologies Overview – Part 1

Technologies overview – part 1 Contents

• On-site systems • Decentralized systems: separate presentation • Activated sludge • Trickling filters • Rotating Biological Contactors • MBR • Anaerobic reactors • Natural systems (ponds, wetlands Household or onsite sanitation

• Simple Pit Toilet • Ventilated Improved Pit (VIP) Toilet • Pour Flush Toilet • Eco-san Toilet Septic tank

•x

Decentralized systems Decentralised: different levels possible

• Decentralised sanitation technology can be applied on all scales: household, small community or complete neighbourhoods Off-site Biological systems

• Attached growth • Suspended growth

• Aerobic • Anaerobic Contents

• On-site systems • Decentralized systems: separate presentation • Activated sludge • Trickling filters • Rotating Biological Contactors • Aerated filters • Anaerobic reactors • Natural systems (ponds, wetlands) Activated Sludge

• Since 20th century • Secondary treatment • Involves treating the liquid part of the wastewater biologically. It is carried out after primary treatment (which removes some of the solid material) • The purpose of this process is to remove the organic matter (and N, P) from the wastewater by bacteria in suspension Characteristics of activated sludge Process

• So far the mostly widely used biological process for the treatment of municipal and industrial wastewaters in developed countries • Microbial community (activated sludge) is highly diverse and competitive Liquid-solids separation, usually sedimentation tank • A recycle system for returning solids removed from the liquid- solids separation unit back to the reactor to maintaining a high concentration of cells • Formation of flocculent settleable solids that can be removed by gravity settling Basic activated sludge process flow sheet Advantages

• Adapted to any size of community (except very small ones) • Good elimination of all the pollution parameters (SS,COD, N, P); • Partially-stabilized sludge • Small area required Disadvantages

• Relatively high capital costs • High energy consumption • Requires skilled personnel and regular monitoring • Sensitivity to hydraulic overloads • Settling property of sludge is not always easy to control (bulking sludge) • High production of sludge Activated sludge performance

• BOD removal (%): 90-98 • Kjeldahl-N removal (%): 80-95 • Total N removal (%): 65-90 • Pathogen removal (orders of magnitude): 1-2 Other important Operating Parameters

 Oxygen supply

 Control and operation of the final settling tank

Functions settling tank • Clarification • Thickening

Sludge settleability is determined by sludge volume index (SVI)

V x 1000 Settling Problem in Activated Sludge Processes

Settling WWTP in Greece facing operational problems due to excessive bulking and foaming of activated sludge

Source: www.hydro.ntua.gr/images/project/AndrBulking.jpg Settling problem Microorganisms in the activated sludge system

Activated sludge floc:

• Bacteria: major component • Fungi: low pH, toxicity, N deficient waste • Protozoa: grazing on bacteria • Rotifers: multicellular organism (help in floc formation) • Organic/ inorganic particles

Activated Sludge, Hoek van Holland, The Netherlands Type of reactor

• Plug-flow reactors • Completely-mixed reactorss

Heiploeg WWTP, The Netherlands

Ecco WWTP, The Netherlands Activated sludge process with chemical P removal

Source: Prof. H Kroiss, Institute for Water Quality, Resource and Waste Management Vienna University of Technology, 2008 Activated sludge process with biol. N removal

Source: Prof. H Kroiss, Institute for Water Quality, Resource and Waste Management Vienna University of Technology, 2008 Contents

 Provision of oxygen to bacteria !! Ventilation !!  Venting of waste gases (CO2 etc.)

Why trickling filters?

Advantages Disadvantages

• Simplicity of operation • Relatively Low BOD Removal (85%) • Resistance to shock loads • High Suspended Solids in the Effluent (20 to 30 mg/l) (“sloughing” • Low biosolids yield of biofilm) • Low power requirements • Very little operational control Factors affecting performance

•Media type and depth •Hydraulic and organic loading •Ventilation •Filter staging •Recirculation rate •Flow distribution Trickling filter media types

• Rock Media – Filter depth 1 to 2.5 m • Rocks 3 to 10 cm in diameter • Heavy so only suitable for small filter depths

• Plastic Media – Filter depth 10 to 13 m • Greater surface area than rocks so more attachment opportunity for bacteria • Much Lighter so suitable for larger filter depths • Larger filter depths means smaller surface areas Trickling filter types using rocks

• Standard or low rate trickling filter • single stage rock media units • loading rates of 1-4 m3 wastewater/m2 filter cross- sectional area-day • large area required

• High rate trickling filter • single stage or two-stage rock media units • loading rates of 10-40 m3 wastewater/m2 filter cross-sectional area-day • re-circulation ratio 1-3 Super rate trickling filter

• synthetic plastic media units • modules or random packed • specific surface areas 2-5 times greater than rock • much lighter than rocks • can be stacked higher than rocks • loading rates of 40-200 m3 wastewater/m2 filter cross-sectional area-day • plastic media depths of 5-10 m Typical trickling filter plant scheme Recirculation

PC Secondary PreTreatment Primary

TF TF Influent PC SC

Disinfection

Biosolids Treatment Disposal Effluent Why Recirculation?

• Reduce strength of the filter influent and/or dilute toxic wastes • Maintain a constant wetting rate • Force sloughing to occur due to increased shear forces. Rotating Biological Contactor Rotating biological contactors

• Consist of 2-4 m diameter disks, closely spaced on a rotating horizontal shaft • Disks are covered with biofilm that rotates in and out of the wastewater to repeatedly wet and aerate the biofilm • Shaft rotates at 1-2 rpm

Film of Microorganisms Rotation

WastewaterWastewater Rotating biological contactors

• Shafts • max. length limited to 9m with 8m occupied by media • Disks (Media) • polyethylene provided in different configurations or corrugation patterns. • Drive systems • rotated by direct mechanical drive units, air-drive • Enclosures • segmented fiberglass-reinforced plastic covers or housed in a building; for protection of plastic media from UV attack, for low temperature control, for protection of equipment, and for control of the buildup of algae in the process • Settling tanks • similar to trickling filter settling tanks • Operating problems • shaft failures, media breakage, bearing failure, and odor problems Flow diagrams for RBCs Factors affecting RBC performance

•Number of stages •Organic loading •Hydraulic loading •Recirculation rate •Submergence •Rotational speed •Oxygen levels RBC design and operational parameters

Treatment level Parameters Combined Separate Secondary nitrification nitrification Hydraulic loading, m3/m2·day 0.08~0.16 0.03~0.08 0.04~0.1 Organic loading 2 gSBOD5/m ·day 3.7~9.8 2.4~7.3 0.5~1.5 2 gTBOD5/m ·day 9.8~17.2 7.3~14.6 1.0~2.9 Maximum loading on first stage 2 gSBOD5/m ·day 19~29 19~29 2 gTBOD5/m ·day 39~59 39~59 2 NH3 loading, gN/m ·day 0.7~1.5 1.0~2.0 Hydraulic retention time, hr 0.7~1.5 1.5~4 1.2~2.9 Effluent BOD5, mg/L 15~30 7~15 7~15 Effluent NH3, mg-N/L < 2 < 2 Energy requirements of RBCs

Unit Process Energy Usage (kWh/yr) Activated Sludge 1,000,000 RBC 120,000 Waste Stabilization Pond 0

Notes: Flow = 3.785 m3/day

Influent BOD5 = 350 mg/L Excludes Pumping and Pretreatment Costs Contents

• What is it? What is a MBR in wastewater treatment?

• A MBR is a process variation on the conventional activated sludge (CAS) process • Suspended Growth System • Clarification (Biosolids Separation) and Biomass Recycle

• Membranes instead of secondary clarifiers (and granular media filtration) to separate the MLSS from the secondary effluent MBR vs. CAS MBR versus CAS process parameters

• MBR process operates over a considerably different range of parameters than the conventional activated sludge process • SRT 5 - 20 days for conventional system 20 - 30 days for MBR (higher) • F/M 0.05 - 1.5 d-1 for conventional system < 0.1 d-1 for MBR (lower) • MLSS 2,000 mg/L for conventional process 5,000 - 20,000 mg/L for MBR (higher)

• Generally, no primary clarification with MBR; practice may change with larger MBRs MBR process for organic matter, N, P removal

effluent

Membrane Influent

Air Anaerobic Anoxic Bioreactor Aerobic • An aerobic compartment for COD degradation & nitrification; • An anoxic compartment for denitrification; • An anaerobic compartment for phosphorus removal. Advantages MBR

• Small footprint • No settlement problems • No further polishing required for disinfection/clarification • No equalisation of hydraulic and organic loadings required Disadvantages

• Membrane surface fouling • Membrane channel clogging • Process complexity • High capital cost • High running costs MBR applications

• Municipal wastewater: • Retrofitting/expansion of existing plant • Placement of submerged membrane modules in aeration basin • Parallel expansion alongside conventional WWTP • Wastewater reuse (helps justify costs) • Strict effluent regulations

• Removal of emerging trace organic compounds • Pharmaceutically Active Compounds • Endocrine Disrupting Compounds • Other recalcitrant MBR process operations MBR

Membrane Sludge return

CAS Influent effluent

Air Bioreactor Side-StreamSide-stream MBR MBR

effluent

Membrane CAS Influent

Air Bioreactor SubmergedSubmerged MBR MBR History

Relatively new technology, rapid expansion: • Sidestream MBR configuration commercialised in 1970’s, niche applications, little market penetration • First submerged MBR installed in 1990 • Exponential growth in installed capacity over last 15 years MBR Process configurations

•Submerged •Sidestream •More recent development •Longest history •Membrane placed in bioreactor •Membrane placed externally •Higher aeration cost •Lower aeration costs •Lower liquid pumping costs •Higher liquid pumping costs; •Lower flux •Higher flux •Less frequent cleaning required •More frequent cleaning require •Lower operating costs •Higher operating costs •Higher capital costs •Lower capital costs Membrane fouling

Three different ways of defining fouling:

• Practically: reversible, irreversible or irrecoverable

• Mechanistically: surface coating, pore plugging

• Material type: chemical nature, particle size, origin Membrane cleaning

Physical Chemical BASE Caustic soda Citric/Oxalic BACKFLUSHING ACIDS • with air CHEMICALLY Hydrochloric/sulphuric • without air ENHANCED Citric/Oxalic BACKWASH RELAXATION OXIDANT Hypochlorite Hydrogen peroxide MBR design

• Obtaining appropriate balance between operational flux, aeration and cleaning, which means: • maximising impact of aeration • facilitating cleaning with minimal downtime and chemicals consumption • providing a high membrane area at a low cost

• Most important of these is aeration efficiency Contents

• Production of energy-rich methane • No energy demand for aeration • No removal of nitrogen and phosphorus (this is an advantage if effluent is to be reused in agriculture) • High organic loading rates can be applied • Suitable for high-strength wastewater (high BOD) • Low production of excess sludge; the digestate is highly stabilized and can easily be dewatered Limitations treating domestic sewage

• Not effective in removing nutrients • Only partially effective in removing pathogens • Difficulties in removing finely dispersed solids • Low activity at temperatures < 10-15 °C • At low temperatures the hydrolysis rate of particulate matter becomes the rate limiting step Low rate/ high rate anaerobic systems

Low rate: • Anaerobic digester • Anaerobic pond High rate • Able to separate hydraulic retention time (HRT) from solids retention time (SRT): higher volumetric loading rates can be applied, and enhanced removal efficiencies can be achieved. Flow sheet anaerobic treatment Application of high rate reactors so far

• Mainly treating industrial wwater • Majority based on the UASB design concept developed by Prof. Lettinga • COD removal efficiency 85-90% • In case of domestic wwtp: T> 15oC, HRT = 4 - 8 hours, Organic loading rate = 1 - 2 kg COD m-3. day-1, COD removal 65 - 80% Basic idea of UASB concept

Flocculent or granular sludge: good settling characteristics

The required good contact is achieved by: • Even feed distribution at the bottom of the reactor • Agitation brought about by the natural biogas production • High upward velocity • Gas Solids Separator installed at the top of the reactor Gas Liquid Solids Separator (GLSS) Device

• Separation of the biogas • To enable the sludge to slide back into the digester compartment • To prevent the wash out of floating granular sludge Feed Inlet System

• The feed inlet distribution system is a crucial part of the reactor • It is important to accomplish optimal contact between sludge and waste water: • To prevent channeling of the waste water through the sludge bed, • To avoid the formation of dead corners in the reactor • The danger of channeling will be bigger at low gas production rates (less than 1 m3/m3 day) Options for treatment of raw or settled municipal- wastewater at temperature < 15oC in high-rate anaerobic systems

• One-step UASB reactor • long HRT is needed 12 - 24 hours • COD removal = 45- 65 %

• Two-step system: • UASB + EGSB (expanded granular sludge bed) • AF (anaerobic filter) + UASB • First step:- HRT = 3 - 4 hours • Second step:- HRT = 4 - 8 hours • COD removal = 50- 70 % Post treatment of the anaerobically treated municipal wastewater

• Removal of pathogens • Removal of the nutrients (depending on the effluent standards)

The most applied systems for the post treatment: • Pond, Duckweed, Wetland • Tricking filter • Rotating biological contactor • Aerated lagoon • Activated sludge process