16 Secondary Treatment

Babita Khosla

17.1 Introduction

Secondary treatment is a biological treatment process for wastewater (or sewage) to achieve a certain degree of effluent quality by using a treatment plant with physical phase separation to remove settles able solids and a biological process to remove dissolved and suspended organic compounds. After this kind of treatment, the wastewater is called secondary-treated wastewater.

The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids. In most cases, secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms mainly bacteria that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products (principally CO2, NH3, and H2O). Several aerobic biological processes are used for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter.

High-rate biological processes are characterized by relatively small reactor volumes and high concentrations of microorganisms compared with low rate processes. Consequently, the growth rate of new organisms is much greater in high-rate systems because of the well-controlled environment. The microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary effluent. The sedimentation tanks used in secondary treatment, often referred to as secondary clarifiers, operate in the same basic manner as the primary clarifiers described previously. The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing.

The common high-rate processes include the activated sludge processes, trickling filters or bio-filters, oxidation ponds, and rotating biological contactors (RBC). A combination of two of these processes in series is sometimes used to treat municipal wastewater containing a high concentration of organic material from industrial sources.

17.2 Activated Sludge

In the activated sludge process, the suspended-growth reactor consisting of an aeration tank or basin containing a suspension of the wastewater and microorganisms is used. The contents of the  aeration tank are mixed vigorously by aeration devices which also supply oxygen to the biological suspension. Aeration devices commonly used include submerged diffusers that release compressed air and mechanical surface aerators that introduce air by agitating the liquid surface. Hydraulic retention time in the aeration tanks usually ranges from 3 to 8 hours but can be higher with high BOD5 wastewaters. Following the aeration step, the microorganisms are separated from the liquid by sedimentation and the clarified liquid is secondary effluent. A portion of the biological sludge is recycled to the aeration basin to maintain a high mixed-liquor suspended solids (MLSS) level. The remainder is removed from the process and sent to sludge processing to maintain a relatively constant concentration of microorganisms in the system. Several variations of the basic activated sludge process, such as extended aeration and oxidation ditches, are in common use, but the principles are similar.

The high concentration of toxic wastes like pesticides, metal plating waste or pH extremes can kill the microbes and affect the activated sludge process.

17.3 Trickling Filters

A trickling filter or bio-filter consists of a basin or tower filled with support media such as lime stone chips, plastic shapes, wooden slats or specially fabricated media. The support media must have large surface area to support bio-film formation. Wastewater is applied intermittently, or sometimes continuously, over the media through perforated spray arm. The microorganisms grow on the support media attached to it forming a biological layer or fixed film. As the waste  water trickles down the organic matter in the wastewater diffuses into the film, where it is metabolized and the treated water is collected in the drain at the base. Oxygen is normally supplied to the microbial film by the natural flow of air either up or down through the media, depending on the relative temperatures of the wastewater and ambient air. Forced air can also be supplied by blowers but this is rarely necessary. The thickness of the biofilm increases as new organisms grow. Periodically, portions of the film ‘slough off the media. The sloughed material is separated from the liquid in a secondary clarifier and discharged to sludge processing. Clarified liquid from the secondary clarifier is the secondary effluent and a portion is often recycled to the biofilter to improve hydraulic distribution of the wastewater over the filter.

The secondary effluent can be again pumped and allowed to pass through the trickling filter if the desired level of BOD reduction is not achieved.

The overloading of trickling filter bed may increase the bio-film thickness leading to anaerobic conditions and possibly clogging of the filter media.

Figure: 2 A Schematic of a complete trickling filter system (Source: http://Trickle_Filter.png: Mbeychok)

17.4 Rotating Biological Contactors

Rotating biological contactors (RBCs) are fixed-film reactors similar to biofilters in that organisms are attached to support media. In the case of the RBC, the support media are slowly rotating discs that are partially submerged in flowing wastewater in the reactor. Oxygen is supplied to the attached biofilm from the air when the film is out of the water and from the liquid when submerged, since oxygen is transferred to the wastewater by surface turbulence created by the discs’ rotation. Sloughed pieces of biofilm are removed in the same manner described for biofilters.

Figure 3: Rotating Biological contactors (Image: https://greenchemie.files.wordpress.com/2013/07/aerobic-21.jpg)

The high-rate biological treatment processes, in combination with primary sedimentation, typically remove 85 % of the BOD and suspended solids originally present in the raw wastewater and some of the heavy metals. The activated sludge generally produces an effluent of slightly higher quality, in terms of these constituents, than biofilters or RBCs. When coupled with a disinfection step, these processes can provide substantial but not complete removal of bacteria and virus. However, they remove very little phosphorus, nitrogen, non-biodegradable organics, or dissolved minerals.

17.5 Natural biological treatment systems: The natural low-rate biological treatment systems are available for the treatment of organic wastewaters such as municipal sewage and tend to be lower in cost and less sophisticated in operation and maintenance. Although such processes tend to be land intensive by comparison with the conventional high-rate biological processes already described, they are often more effective in removing pathogens and do so reliably and continuously if properly designed and not overloaded. The various types of natural biological treatment systems include

Wastewater stabilization ponds Overland treatment of wastewater/ Constructed treatment wetlands Macrophyte treatment Nutrient film technique

Among the natural biological treatment systems available, stabilization ponds and land treatment have been used widely around the world and a considerable record of experience and design practice has been documented. The nutrient film technique is a fairly recent development of the hydroponic plant growth system with application in the treatment and use of wastewater.

17.5.1 Wastewater Stabilization Ponds:

Stabilization ponds are an aerated basins and important part of the natural treatment ways. The desired treatment effect is achieved by physical, chemical and biological processes occurring in the aquatic environment in the presence of water and wetland biocoenosis (bacteria, phytoplankton, and zooplankton), higher vegetation and organisms.

Figure 4: Waste water stabilization pond (Image: http://www.thewatertreatments.com/waste-stabilization-ponds/)

They can be divided into the categories according to the treatment technologies.

1. Anaerobic ponds
2. Facultative Ponds
3. Aerobic ponds

For the most effective treatment all the three wastewater stabilization ponds should be linked in a series with effluent being transferred from anaerobic to facultative and finally to the aerobic pond (also called maturation pond).

Figure 5 : Waste water Stabalization Ponds in Series  Source: Tilley et al. 2014

The anaerobic pond act as primary treatment stage and reduces the organic load by sedimentation. Then subsequent anaerobic digestion occurs inside the settled accumulated sludge. The organic carbon is converted to methane gas and upto 60% removal of BOD takes place here.

The effluent from the anaerobic pond is transferred to the facultative pond where further BOD removal takes place. The top layer receives oxygen from natural diffusion, wind mixing and algae driven photosynthesis while the lower layer is deprived of oxygen. So, Both aerobic and anaerobic organisms work together to achieve BOD reduction.. The water from these enters into the aerobic ponds which are designed for pathogen removal. They are also called maturation or finishing ponds. It is the last step and the shallowest pond may range in depth from 1.5 to 5.0 meters and ensuring final level of treatment. Wastewater stabilization ponds achieve 80-90% BOD removal with retention times of 1-10 days. They are low cost for operation and maintenance but high removal of BOD and pathogens. The treated water still contains nutrients and is appropriate for reuse in agriculture but not for direct recharge in surface waters.

17.5.2 Macrophyte treatment

The natural and artificial wetlands and marshes have been used to treat raw sewage or partially treated effluents. The natural wetlands are usually unmanaged whereas artificial systems are designed to maximise performance by providing the optimum conditions for the growth of macrophytes.

Maturation ponds which incorporate floating, submerged or emergent aquatic plant species are termed macrophyte ponds and these have been used in recent years for upgrading effluents from stabilization ponds. Macrophytes take up large amounts of inorganic nutrients (especially N and

1. P) and heavy metals (such as Cd, Cu, Hg and Zn) as a consequence of the growth requirements and decrease the concentration of algal cells through light shading by the leaf canopy and, possibly, adherence to gelatinous biomass which grows on the roots.

Floating macrophyte systems utilizing water hyacinth and receiving primary sewage effluent have been used to achieve secondary treatment effluent quality.

Figure 6: Constructed wetland (Image: https://www.pinterest.com/patterndance/constructed-wetlands/)

Floating macrophyte species, with their large root systems, are very efficient at nutrient stripping. Several genera have been used including Phragmites communis, Salvinia, Spirodella, Lemna and Eichornia (O’Brien 1981). In such macrophyte pond systems, apart from any physical removal processes which might occur (especially sedimentation) the aquatic vascular plants serve as living substrates for microbial activity, which removes BOD and nitrogen, and achieves reductions in phosphorus, heavy metals and some organics through plant uptake. The basic function of the macrophytes in the latter mechanism is to assimilate, concentrate and store contaminants on a short-term basis. Subsequent harvest of the plant biomass results in permanent removal of stored contaminants from the pond treatment system. The nutrient assimilation capacity of aquatic macrophytes is directly related to growth rate, standing crop and tissue composition. The potential rate of pollutant storage by an aquatic plant is limited by the growth rate and standing crop of biomass per unit area.

Fly and mosquito breeding is a problem in floating macrophyte ponds but this can be partially alleviated by introducing larvae-eating fish species such as Gambusia and Peocelia into the ponds. It should be recognized that pathogen die-off is poor in macrophyte ponds as a result of light shading and the lower dissolved oxygen and pH compared with algal maturation ponds. In their favour, macrophyte ponds can serve a useful purpose in stripping pond effluents of nutrients and algae and at the same time produce a harvestable biomass. Floating macrophytes are fairly easily collected by floating harvesters. The harvested plants might be fed to cattle, used as a green manure in agriculture, composted aerobically to produce a fertilizer and soil conditioner  or can be converted into biogas in an anaerobic digester, in which case the residual sludge can then be applied as a fertilizer and soil conditioner.

The key features of such macrophyte treatment systems are:

Wastewater BOD and nitrogen are removed by bacterial activity; aerobic treatment takes place in the rhizosphere, with anoxic and anaerobic treatment taking place in the surrounding soil. Oxygen passes from the atmosphere to the rhizosphere via the leaves and stems of the reeds through the hollow rhizomes and out through the roots.

Suspended solids in the sewage are aerobically composted in the above-ground layer of vegetation formed from dead leaves and stems.

Nutrients and heavy metals are removed by plant uptake. The growth rate and pollutant assimilative capacity of macrophytes are limited by the culture system, wastewater loading rate, plant density, climate and management factors. As emergent macrophytes have more supportive tissue than floating macrophytes, they might have greater potential for storing the nutrients over a longer period. Consequently, frequent harvesting might not be so necessary to achieve maximum nutrient removal although harvesting above-ground biomass once a year should improve overall nutrient removal efficiency.

17.5.3 Nutrient film technique

The nutrient film technique (NFT) is a modification of the hydroponic plant growth system in which plants are grown directly on an impermeable surface to which a thin film of wastewater is continuously applied. Root production on the impermeable surface is high and the large surface area traps and accumulates matter. Plant top-growth provides nutrient uptake, shade for protection against algal growth and water removal in the form of transpiration, while the large mass of self-generating root systems and accumulated material serve as living filters. The following mechanisms have been hypothesized by Jewell et al. in 1983, taking place in three plant sections:

Roughing or preliminary treatment by plant species with large root systems capable of surviving and growing in a grossly polluted condition. Large sludge accumulations, anaerobic conditions and trace metal precipitation and entrapment characterize this mechanism and a large portion of wastewater BOD and suspended solids would thereby be removed. Nutrient conversion and recovery due to high biomass production.

Wastewater polishing during nutrient-limited plant production, depending on the required effluent quality. A three year pilot-scale study by Jewell et al. (1983) proved this to be a viable alternative for sewage treatment. Reed canary grass was used as the main test species and resulted in the production of better than secondary effluent quality at an application rate of 10 cm/d of settled domestic sewage and synthetic wastewater. The highest loading rates achieved were equivalent to treating the sewage generated by a population of 10,000 on an area of 2 ha. Plants other than reed canary grass were also tested and those that flourished best in the NFT system were:

cattails, bulrush, strawflowers, Japanese millet, roses, Napier grass, marigolds, wheat and phragmites.

Figure 7: Nutrient film technique variation of hydroponic plant production systems (Image: http://www.tropicannahorticulture.com/media/images/drip(1).jpg)

References:

Tilley E.; Ulrich L.; Luethi C.; Reymond P.; Zurbruegg C. (2014): Compendium of Sanitation Systems and Technologies. 2nd Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag).