1. ERT 417/4 WASTE TREATMENT IN BIOPROCESS INDUSTRY CH 14 – Thickening, Aerobic & Anaerobic Digestion Prepared by: Pn. Hairul Nazirah Abdul Halim

3. Sludge: Materials containing large amount of water and a small part of solids (1-6%) Needs to be treated in a correct manner to avoid environmental contamination Sludge also contains organics and inorganic component and other forms of contaminates The sources and characteristics of solids and sludge depend on the type of plant and method of operation INTRODUCTION: Sludge Treatment

4. OBJECTIVE OF SLUDGE TREATMENT To stabilize organic content in the sludge To reduce the volume of sludge (Since water constitutes most of the sludge, water removal can reduce most of the volume) To remove pathogenic organisms in sludge (causes of disease) To acquire valuable materials from sludge, especially useful gases, and also the sources of compost

5. METHOD OF SLUDGE THICKENING A physical process for volume reduction Increase the solids content of sludge by removing portion of the liquid fraction Reduces sludge volume to facilitate ease in handling, treatment and disposal Method of thickening: - Co-settling thickening - Gravity thickener - Air Flotation - Centrifugation - Gravity belt - Rotary drum

6. Occurrence of thickening methods in solids processing

7. Method of thickening: Gravity Thickener Resembles a circular sedimentation tank and has vertical pickets attached to the sludge scraper arm Dilute sludge is fed to the center of feed well, and allowed to settle and compact As the scraper arm moves slowly the pickets gently stir the sludge to release trapped water and air from it The thickened sludge is withdrawn from the bottom and the supernatant is recycled to the treatment plant inlet Sludge can be thickened to more than 10% solids in a gravity thickener Designed based on thickener overflow rate and solids loading

8. Schematic diagram of a gravity thickener Method of thickening: Gravity Thickener

9. Schematic diagram of a gravity thickener Method of thickening: Gravity Thickener

10. Typical concentrations of unthickened and thickened sludge and solids loadings for gravity thickeners

11. Air is forced into solution under pressure and then mixed with sludge in the influent The mixture enters a floatation tank and the rising air bubbles carry the sludge flocs to the liquid surface The layer of thickened sludge formed on the liquid surface is skimmed off The skimmed effluent exits as underflow Method of thickening: Dissolved Air Floatation (DAF)

12. Sludge can be thickened to about 6% solids Higher loadings can be used with DAF than gravity thickeners due to rapid separation of solids from the wastewater Method of thickening: Dissolved Air Floatation (DAF)

13. Typical dissolved air floatation unit used for thickening waste activated sludge

14. Thickening (and dewatering) by centrifugation involves the settling of sludge particles under the influence of centrifugal forces. The basic type of centrifuge used for sludge thickening is the solid-bowl centrifuge. Method of thickening: Centrifugation

15. Schematic diagram of a centrifuge used for sludge thickening

16. Stemmed from the application of belt-press for sludge dewatering In belt-press dewatering, sludge thickening occurred in the gravity drainage section of the press A gravity belt moves over rollers driven by a variable-speed drive unit The sludge is conditioned with polymer and fed into a feed box at one end The water drains through the belt as the concentrating sludge is carried toward the discharge end of the thickener Method of thickening: Gravity Belt Thickener

17. Schematic diagram of a gravity belt thickener Method of thickening: Gravity Belt Thickener

18. SLUDGE STABILIZATION Sludge are stabilize to reduce pathogens, eliminate offensive odors and reduce or eliminate potential for putrefaction Sludge stabilization methods include: –Lime stabilization –Heat treatment –Anaerobic digestion –Aerobic digestion –Composting –Disinfection –Wet air oxidation

19. Method of Sludge Stabilization: Anaerobic Digestion Decomposition of organic and inorganic matter in the absence of molecular oxygen. Principal function: convert as much of the sludge as possible to end products such as liquids, gases, and as little residual biomass as possible Produces considerably less biomass than aerobic processes

20. Method of Sludge Stabilization: Anaerobic Digestion The sludge undergoes 1. hydrolysis 2. fermentation/acidogenesis (mainly formation of organic acid) 3. methanogenesis (mainly formation of methane and carbon dioxide) Anaerobic organisms function within a narrow pH range (6.5- 7.5) Organisms sensitive to temperature, food supply and shock loading. Product gases: 65-70% methane, 25-30% CO 2 and trace amounts of other gases.

21. Advantages lower biomass yield fewer nutrients required energy production (methane) less energy required less biological sludge production Disadvantages longer start up time to develop necessary biomass inventory may require alkalinity and/or specific ion addition may require further treatment with an aerobic treatment process to meet discharge requirements Biological nitrogen and phosphorus removal is not possible Potential for production of odors and corrosive gases

22. Can be either standard-rate (single stage) or high-rate (two- stage) digestion Standard rate anaerobic digester - Consist of closed tanks with air covers - Completely mixed, continuously-flow model - Often use for economic reasons or simplicity of operation

24. -The conical bottom facilitates sludge withdrawal -‘floating’ cover accomodates volume changes due to sludge additions and withdrawals -Some mixing occurs in the zone of active digestion and in the supernatant because of withdrawal and return of heated sludge -Sludge is fed on an intermittent basis -Solid retention time (SRT) – 30 - 90 days -Supernatant is withdrawn and return to secondary treatment unit -Digested sludge is accumulated in the bottom and will be removed to disposal facilities

25. Method of Sludge Stabilization: Anaerobic Digestion Two Stage High Rate Anaerobic Digester more efficient and often require less volume than single stage digester the contents are mechanically mixed in first state reactor to ensure better contact between the organics and microorganism the unit is heated to increase the metabolic rate of the microorganism, thus speeding up the digestion process optimum temperature is around 35°C Solid retention time (SRT) – 10-20 days Little gas is generated in the second-stage

26. Consequently, the second stage reactor is usually covered and equipped for gas recovery Second stage reactor is not heated -Supernatant is withdrawn and return to secondary treatment unit -Digested sludge is accumulated in the bottom and will be removed to disposal facilities

28. Method of Sludge Stabilization: Aerobic Digestion The decomposition of organic matter and inorganic matter in the presence of molecular oxygen. Oxidation process produces CO 2, water, ammonia SRT for activated sludge only – 15 – 20 days SRT for activated sludge + primary – 20 – 25 days Use to treat: 1.waste-activated sludge only 2.Mixtures of waste-activated sludge or trickling filter and primary sludge 3.Waste sludge from extended aeration plants

29. Examples of aerobic digesters: (a) Batch operation type with air addition (b) Continuous operation type with air addition Method of Sludge Stabilization: Aerobic Digestion

30. Advantages lower BOD concentrations in supernatant liquor production of odourless, humuslike, biologically stable end product suitable for digesting nutrient-rich biosolids Disadvantages high power cost (supply O 2 ) useful by product such as methane is not recovered. Process is significantly affect by process parameters such as temperature, aeration device, conc. Of feed solids, etc.