Question

14. (a) Explain with specific details why the effluents from the municipal waster water treatment plants (MWTPs) that use tertiary treatment process pose lower risks to the receiving environment compared to the plants that use primary or secondary treatment process. (b) Explain with appropriate documented examples how the effluents from MWTPs affect aquatic ecosystems and why. (c) Compare the benefits and limitations of end of pipe whole effluent toxicity evaluation and Environmental Effects Monitoring approach.

Answer 1

Question: (a) Explain with specific details why the effluents from the municipal waster water treatment plants (MWTPs) that use tertiary treatment process pose lower risks to the receiving environment compared to the plants that use primary or secondary treatment process

Answer:  The treatment of wastewater (and sewage) spreads across three main stages: Primary Treatment, Secondary Treatment and finally the Tertiary Treatment. The use tertiary treatment process pose lower risks to the receiving environment . Tertiary treatment process is used to improve the quality of the effluent which has resulted from primary and secondary treatment processes. After the tertiary treatment, the water can be safely discharged into the environment (water bodies like rivers and lakes etc.) And used for agricultural, industrial and, in many cases, domestic purposes.

As the wastewater reaches the tertiary treatment stage, it still has residual suspended matter and fine particulates. Further, it has a relatively high level of nutrients such as nitrogen and phosphorus and has microbes and odor in it. During tertiary treatment process, different methods are used to remove all these contaminants and properties from wastewater.

The first stage of tertiary treatment is filtration which helps primarily to remove residual suspended matter in wastewater. Sand filtration is the usual method that is used for this purpose. In some cases, residual toxins may be present in wastewater and to filter them out, activated carbon is used to adsorb the toxins and remove them from wastewater.

After filtration, some fine particulate matter may still remain in the wastewater. To treat the wastewater further, it is transferred to lagoons with and filter feeders such as Daphne. In the lagoon’s aerobic and biologically enriched environment, further removal of fine particulate matter takes place. The wastewater may still have high nutrient concentration. Nitrogen and phosphorus are usually found in high concentration in wastewater and this, if untreated and released into the natural water environment, can cause excessive growth, consequent death (due to nutrient and space limitation) and eventual decomposition of algae. As the dead algae due to bacterial action, the amount of oxygen becomes much less than is necessary for other aquatic life to survive. Therefore, reduction of nitrogen and phosphorus concentrations is critical before wastewater release into natural water bodies. Both nitrogen and phosphorus can be removed using biological processes involving different bacteria. Nitrogen in the form of ammonia is first oxidized to form nitrates and then nitrates to nitrogen gas which is released to the environment. On the other hand, phosphorus can be removed biologically as well by chemical precipitation with salts of iron, aluminium, or lime. accumulating bacteria can accumulate high percentage of phosphorus and the biomass thus generated can further be of good value as fertilizer. Similarly, phosphate-rich sludge which is produced as a result of precipitation during chemical treatment for removal of phosphorus has good value as fertilizer.

The final steps in tertiary wastewater treatment, before release into the environment, are removing any undesirable microbes through the process of disinfection and also removal of odors from wastewater. The clear and less cloudy nature of wastewater at this stage is critical for efficiency and effectiveness of the disinfection process. Several disinfection agents can be used depending on wastewater condition (pH, clarity etc.) And among them chlorine, ozone, ultraviolet (UV) light are most common. These have their advantages and disadvantages. Chlorine is a time-tested effective disinfection agent. However, long term use and unregulated dosage can lead to generation of chlorinated-organic compounds from chlorination of residual organic matter in water. Such compounds may be carcinogenic in nature. Therefore, a de-chlorination step is important if the water is to be discharged into larger aquatic bodies. Ultraviolet light treatment for disinfection does not leave any residues or compounds behind in the water but for it to be effective, the wastewater must be clear. Proper lamp and equipment maintenance is necessary for long term usage of UV light. Ozone is also a very effective disinfection agent and does not need any storage place as it can be generated on site using the necessary equipment for passing oxygen through high voltage current.

Odor in wastewater is mainly a result of release of gases, such as hydrogen sulfide, from anaerobic processes in wastewater. Carbon reactors, regulated amounts of chlorine, hydrogen peroxide, calcium nitrate etc. Can be effectively used to manage hydrogen sulfide levels and thus reduce odors from wastewater.

Eventually, the wastewater after the tertiary treatment is fitting, after necessary tests, to be released into the environment for further re-use.

Question: (b) Explain with appropriate documented examples how the effluents from MWTPs affect aquatic ecosystems and why

Answer: Industrial wastes often pass through the same sewer treatment facilities as domestic wastes. Industrial waste often contains a variety of chemicals and may also contain heavy metals like lead, mercury, cadmium and arsenic. Not all of these chemicals are completely removed in sewage treatment plants, so the chemicals are released into rivers, lakes and marine waters. In addition, some waste may be released or spilled into aquatic ecosystems without any treatment. The effects of sewage pollution on marine life impact organisms throughout the food chain.

Heavy metals build up in fish tissues as the fish consume plankton, algae and smaller prey containing the metals. This process is called biomagnification. As other animals, including humans, eat these fish, the heavy metals can reach sufficient concentrations to poison the consumer. These heavy metals may accumulate in toxic amounts for fish as well.

Control of releases of industrial sewage like petroleum products, radioactive waste and persistent organic pollutants has improved, with oily wastes reduced by 90 percent between the 1980s and 2006. These pollutants caused immediate and long-term effects on ecosystems by poisoning or smothering plankton, plants and animals.

During recent years, increasing incidences of summer droughts - likely driven by climate change - reduced the dilution potential of low-order streams for secondary treated wastewater also in temperate Europe. Despite the potential risks to ecosystem integrity, there is a paucity of knowledge regarding the effects of different wastewater dilution potentials on ecosystem functions. The present study investigated the implications of secondary treated wastewater released into a third-order stream (Queich, southwest Germany) during a season with low dilution potential (summer; ~90% wastewater) as compared to a season with high dilution potential (winter; ~35% wastewater) in terms of leaf litter decomposition and macroinvertebrate communities. Adverse effects in macroinvertebrate mediated leaf mass loss (~65%), gammarids' feeding rate (~80%), leaf associated fungal biomass (>40%) and shifts in macroinvertebrate community structure were apparent up to 100 and 300 m (partially 500 m) downstream of the wastewater treatment plant effluent during winter and summer, respectively. In addition, a Gammarus fossarum laboratory feeding trial demonstrated the potential of powdered activated carbon to reduce the ecotoxicity of released wastewater. These results urge the development and evaluation of adequate management strategies, e.g. the application of advanced wastewater treatment technologies, to protect the integrity of freshwater ecosystems, which is required by the European Water Framework Directive - also considering decreasing dilution potential of streams as projected by climate change scenarios.

Question: Compare the benefits and limitations of end of pipe whole effluent toxicity evaluation and Environmental Effects Monitoring approach.

Answer: The End of Pipe or EOP treatment is a system that processes the waste before releasing it on the environment. This includes chemical treatment, recycling, and waste burning.

Benefits.

1. The facility that introduced end of pipe treatment helps to minimize waste disposal and reduces its impacts on the environment.

2. End-of-pipe treatment is a  closed loop operation in which all excess materials are recycled back into the process.

3. End-of-pipe treatments are implemented after production processes to modify the end products and can be operated independently of the production process. This approach reduces the direct release of some pollutants in order to comply with existing environmental regulations.

Limitations:

1. One limitation of this system is that this diminishes the awareness of workers about waste. It diminishes workers' awareness of waste and ability to identify its root causes and it reduces incentives to eliminate waste.

2. EOP are not adequate to allow an efficient use of limited resources. They causes greater consumption of material and energy, more capital expenditure and more work hours compared to measures taken at source. Their operating costs are simply added to the output production costs, which will result in an increase in the total manufacturing costs.