Prepared by: Pn. Hairul Nazirah Abdul Halim ERT 417/4 WASTE TREATMENT IN BIOPROCESS INDUSTRY CH 10 – Anaerobic Suspended & Attached Growth Process Prepared by: Pn. Hairul Nazirah Abdul Halim

The purpose of this chapter – to present anaerobic treatment processes used to remove organic material from liquid streams.

Rationale for Anaerobic Treatment Advantages of Anaerobic Treatment Processes: Energy Considerations Lower Biomass Yield Fewer nutrients required

1. Energy Considerations Net energy producers instead of energy users

2. Lower Biomass Yield Sludge processing and disposal cost reduced greatly. 3. Fewer Nutrients Required Aerobic process require nutrients to support the growth. Anaerobic need less nutrient because less biomass is produced.

Disadvantages of Anaerobic Treatment Processes Operational Considerations Need for Alkalinity Addition Need for Further Treatment

Operational Considerations Longer start-up time (months for anaerobic versus days for aerobic processes) Sensitivity to possible toxic compounds Potential for odor production Corrosiveness of the digester gas However, with proper w/w characterization and process design these problems can be avoided/managed.

2. Need for Alkalinity Addition 2000 to 3000 mg/L CaCO3 may be needed to maintain an acceptable pH with high CO2 concentration. Cost to purchase alkalinity 3. Need for Further Treatment May require further treatment with aerobic process to meet discharged requirements.

Examples of types of w/w treated by anaerobic processes: Alcohol distillation Breweries Chemical manufacturing Dairy and cheese processing Domestic w/w Fish and seafood processing Pulp & paper Soft drink beverages Sugar processing Pharmaceuticals

General Design Considerations for Anaerobic Treatment Processes Characteristic of the Wastewater The presence of toxic streams, flow variations, inorganic conc., seasonal load variations. 2. Flow and Loading Variations Wide variation in influent flow and organic loads can upset the balance between acid fermentation and methanogenesis. Lower pH inhibit methanogenesis

3. Organic concentration and temperature Optimal biological rxtn: 25 – 35 oC COD > 1500 to 2000 mg/L are needed to produce sufficient methane to heat w/w without external heat source. Aerobic treatment can be applied at lower temp (10-20oC) - slower rxtn rates - longer SRT - larger reactor volumes - lower organic COD loading is needed - degradation of long chain fatty acids is limited

4. Fraction of Nondissolved Organic Material High solid conc., are treated in suspended growth reactor than by upflow and downflow attached growth processes 5. Wastewater Alkalinity Alkalinity in the range 2000-4000 mg/L as CaCO3 typically required to maintain pH at/or near neutral. The level of alkalinity needed is seldom available in the influent w/w Requirement to purchase chemical to control pH – impact on economic

6. Solid Retention Time (SRT) SRT larger than 20d are needed for anaerobic processes at 300C.

Anaerobic Suspended Growth Processes 3 types of anaerobic suspended growth processes: 1. Complete-mix suspended growth anaerobic digester 2. Anaerobic contact process 3. Anaerobic sequencing batch reactor

Complete-mix suspended growth anaerobic digester SRT in the range 15 – 30d Without sludge recycle – for high conc. Of solid or extremely high dissolved organic conc.

View of typical pilot plants : (a) pilot-scale anaerobic reactors at a food-processing facility

2. Anaerobic contact process Overcomes the disadvantaged of a complete-mix process without recycle Biomass is separated and returned to the contact reactor

3. Anaerobic sequencing batch reactor (ASBR) Four step: Feed, React, Settle and Decant

Attached Growth Anaerobic Processes Upflow attached growth anaerobic treatment reactors differ by type of packing used and the degree of bed expansion. 3 types: Upflow packed-bed reactor Anaerobic expended bed reactor Fluidized bed anaerobic reactor

Upflow packed-bed reactor The packing is fixed and the w/w flows up Suitable for w/w with low suspended solid conc. Advantages: - high COD loadings - relatively small reactor volumes - operational simplicity Limitations: - cost of packing material - maintanance associated with solids accumulation - possible packing plugging

2. Upflow Attached Growth Anaerobic expended bed reactor Packing material – silica sand (0.2-0.5 mm) Smaller packing provide greater surface area per unit volume Packing void fraction 50% upflow liquid velocity – 2 m/h Advantages and limitations are similar to fluidized bed reactor.

3. Fluidized bed anaerobic reactor Packing material – sand, anion and cation exchange resins and activated carbon Packing size 0.3 mm sand Operate at higher upflow liquid velocity – 20 m/h Advantages – ability to provide high biomass conc. - high biomass loading - high mass transfer characteristic - minimal space requirements Disadvantages: - pumping power required to operate fluidized bed - cost of reactor packing - length of start up time