Biological/Secondary Wastewater Treatment

1. Biological/Secondary Wastewater Treatment Dr.M.Vasanthy Assistant Professor Department of Environmental Biotechnology Bharathidasan University Tiruchirappalli - 24

2. Objectives of wastewater treatment are as follows: To reduce the organic content (reduction of BOD) To remove/reduce the nutrients i.e., N,P To remove/inactivate pathogenic microbes

3. Wastewater treatment • Primary treatment includes • Sedimentation and screening of large debris • Secondary treatment focuses on • Biological and chemical treatment • Tertiary treatment refers to • Further chemical treatment

4. Wastewater treatment • There are three levels of wastewater treatment: primary, secondary, and tertiary (or advanced). • Primary treatment generally removes about 60 percent of total suspended solids and about 35 percent of BOD; however dissolved impurities are not removed. • It is usually used as a first step before the biological or  secondary treatment.

5. Schematic representation of Wastewater treatment

6. Secondary treatment • Secondary treatment generally removes the soluble organic matter. • It also removes the suspended solids too. It is usually accomplished by biological processes in which microbes consume the organic impurities as food, convert them into carbon di oxide, water, and energy for their own growth and reproduction. • There are certain biological treatment methods namely, the trickling filter, the activated sludge process, oxidation pond, lagoons, rotating biological contactor etc., • This treatment removes more than 85 percent of both suspended solids and BOD.

7. Trickling filters • Trickling filters are one of the conventional aerobic biological wastewater treatment units. • The advantage of all these systems is that they are compact and that they efficiently reduce organic matter (JENSSEN et al. 2004). • The trickling filter consists of a cylindrical tank and is filled with a high specific surface area material, such as rocks, gravel, or special plastic filter media. • A high specific surface provides a large area for biofilm formation. Organisms that grow in the thin biofilm over the surface of the media oxidize the organic load of the wastewater to carbon dioxide and water, while generating new biomass too.

8. The incoming pre-treated wastewater is ‘trickled’ over the filter, e.g., with the use of a rotating sprinkler. • Hence, the filter media goes through cycles of being dosed and exposed to air. • The removal/reduction of organic impurities occurs by the microbial action (UNEP & MURDOCH UNIVERSITY 2004). • Therefore trickling filters are also called or biological filters to emphasise that the filtration needs the presence of Zoogleal film.

9. Design Considerations • The filter is usually 1 to 2.5 m deep. • Oxygen is obtained by direct diffusion from air into the filter and the biological film (UNEP, 2004) from the bottom through a spontaneous airflow due to temperature difference. • Therefore, both ends of the filter should be ventilated (TILLEY et al. 2008),). • The primary factors that must be considered in the design of trickling filtersinclude • the type of filter media to be used • the spraying system, and • the configuration of the under-drain system

10. Filter media • The ideal filter material is normally low-cost and durable, has a high surface to volume ratio, is of less weight and allows air to circulate. • Crushed rock or gravel is the cheapest option. Specially manufactured plastic media, is also used. • The particles should be uniform and 95% of them should have a diameter between 7 and 10 cm. • A material with a specific surface area between 45 and 60 m2/m3 for rocks and 90 to 150 m2/m3 for plastic packing is normally used. Larger pores (as in plastic packing) are less prone to clogging and could provide good air circulation.

11. Trickling Filter systemsource: https://www.sswm.info/water-nutrient-cycle/wastewater • The underdrains provide a passageway for air at the maximum filling rate. A perforated slab supports the bottom of the filter, allowing the effluent and excess sludge to be collected. • The trickling filter is usually designed with a recirculation pattern for the effluent to improve wetting and flushing of the filter material.

12. With time, the biomass (Zoogleal layer/biomass) will grow thick and the attached layer will be deprived of oxygen; it will enter an endogenous state, will lose its ability to stay attached and will slough off (U.S. EPA 2000). High-rate loading conditions will also cause sloughing. • Then the cleaning of the Zoogleal mass becomes mandatory. Zoogleal layer of film refers the complex population of organisms that form a slime growth on the trickling filter media and break down the organic matter in wastewater. These slimes consist of living organisms feeding on the wastes in wastewater, dead organisms, silt, and other debris too. • When the microorganismsfall off the medium and are carried with the effluent, this process is known as sloughing (U.S. EPA 2000)

13. The under-drain system permits transporting these solids to a clarifier, where the solids settle and get separated from the treated effluent. • The collected effluent would be clarified in a settling tank to remove any biomass that may have dislodged from the filter. It could then be discharged to surface waters or used for irrigation (ECOSANRES 2008). • Trickling filters can be combined in decentralised wastewater treatment systems .Although trickling filters are more easily operated and consume less energy than activated sludge processes, they have a lower removal efficiency for solids and organic matter, they are more sensitive to low air temperatures, and can become infested with flies and mosquitoes (UNEP et al. 2004).

14. Treatment capacity • A BOD reduction of 60 to 85 % can be expected with loading rates of 1 kg BOD/m3/day (SASSE & BORDA 1998; U.S.EPA 2000; UNEP 2004; WSP 2008;). • Bacterial reductions have been reported to be 1 to 2 logs of faecal Coliforms (UNEP 2004), respectively 60 to 90 % of total Coliforms (WSP 2008). • Physical adsorption of virus on the biofilm or elimination by predation are additional factors in pathogen elimination in trickling filters (STRAUSS n.y.)

15. Activated Sludge Process • The term activated sludge refers to suspended aerobic sludge consisting of flocs of active bacteria, which consume and remove aerobically biodegradable organic substances from screened or screened and pre-settled wastewater. • Activated sludge systems can treat greywater, faecal sludge and industrial wastewater as long as the pollutants to be treated are biodegradable.

16. Schematic of the aeration tank and secondary settling tank (clarifier) of an activated sludge system. Source: Tilley et al. (2014)

17. The process • Different configurations of the activated sludge process can be employed to ensure that the wastewater is mixed and aerated in an aeration tank. • Aeration and mixing can be provided by pumping air or oxygen into the tank or by using surface aerators. • The microorganisms oxidize the organic carbon in the wastewater to produce new cells, carbon dioxide and water. Although aerobic bacteria are the most common organisms, facultative bacteria along with higher organisms can also be present. • The flocs (agglomerations of sludge particles), which form in the aerated tank, can be removed in the secondary clarifier by gravity settling.

18. Activated sludge processsource: https://www.britannica.com/technology/activated-sludge-

19. Detailed Treatment Process • After screening sand and similar heavy particles are removed next in a grit chamber where they settle to the ground. • Smaller solids are removed in a settling or sedimentation tank. • In this unit, the wastewater spends more time (about one hour) to allow for a good separation. • The sludge from this mechanical primary treatment (including screening and settling in the grit chamber and the sedimentation tank) is called primary sludge and, as all excess sludge, requires an advanced further treatment chain.

20. After this primary treatment, the main unit containing the activated sludge follows. The pre-treated wastewater is mixed with the concentrated underflow activated sludge from the secondary clarifier in an aerated tank. • Aeration is provided either by mechanical surface agitators or by submerged diffusers of compressed air. Aeration provides oxygen to the activated sludge and at the same time thoroughly mixes the sludge and the wastewater (UNEP & MURDOCH 2004). • During aeration and mixing, the bacteria form small clusters or flocs (TILLEY et al. 2008). Under these conditions, the bacteria in the activated sludge degrade the organic substances in the wastewater. They use the organic substance for energy, growth and reproduction. The end products are carbon dioxide (CO2), water (H2O) and new cells.

21. The settled microorganisms (the activated sludge) are then recycled to the head end of the aeration tank to be mixed again with wastewater and continue to grow and form new sludge and to degrade organics. • To maintain an optimal amount of sludge in the system, the rate of recirculation of settled sludge varies from 20 to 100%. • Excess sludge produced each day (waste activated sludge) must be processed in a further treatment chain together with the sludge from the primary treatment facilities. • A conventional excess sludge treatment chain consists in anaerobic digestion, thickening, incineration and the safe disposal, e.g. in a landfill.

22. The effluent from a properly designed and operated activated-sludge plant is of high quality, usually having BOD and TSS concentrations equal to or less than 10 mg/L (CRITES & TCHOBANOGLOUS 1998). • The removal of both, biological oxygen demand (BOD) and suspended solids (TSS) generally lies within 80 to 100% depending on the influent concentrations, the system set-up and temperature (UNEP 2004; SANIMAS 2005; WSP 2008).

23. Summary • The wastewater refers to the discharged water from the residential areas called the sewage water or grey water and the industrial wastewaters. • The Primary treatment and the secondary treatment offered to such wastewate has been discussed .

24. Thank you