Tom Flashcards
Describe the water cycle and the water resources that are available to us around the planet. Indicate the sources and sinks of water and the flows between them.
Hydraulic cycle is the continuous recirculatory process that links together water in the atmosphere, in the oceans, and on land. Water flux in and out of oceans into the atmosphere occurs through evaporation and precipitation, in a balanced process. In addition, evaporation of water takes place on land, and can combine with excess evaporation from oceans to form land precipitation. This then runs off into lakes, streams and oceans, or can filter into soil and rock where it moves as groundwater.
97.5% of global water sources are contained in oceans, whilst 2.5% is freshwater. Of this 2.5%, 70% is contained in glaciers, with approximately 30% contained in groundwater, meaning that only 0.01% of all the water on Earth is available as a usable supply for ecosystems and humans.
However, access to this water is not equal. Currently, 1/9 people do not have access to clean drinking water. Asia and the Middle East are home to 60% of the world’s population, but only have access to 36% of the world’s river run off. On the contrary, South America is home to only 6% of the global population, and has access to 26% of its river runoff. Consequently, countries in Asia and the Middle East may have to manage water resources more carefully. Countries that can afford to, invest in desalination plants as a means of providing drinking water to residents.
It is estimated that by 2025, 2/3 of the populations will live in water stressed conditions. Changes in patterns of water availability are largely due to anthropogenic climate change, which results in the melting of the ice caps, which results in the melting of ice caps and subsequent rising of sea levels. A further consequence is the increased risk of flooding in some areas, and droughts in others, the latter of which can occur as a result of increased evaporation rates and plant respiration, which in turn reduces the amount of water held in plants and soil. In addition, higher average temperatures allow the air to hold more water, which can result in dry spells followed by brief yet heavy rainfall. Furthermore, warmed up bodies of water can result in detrimental algal blooms.
Another impact of human activity on water quality and demand is the negative implications arising from water pollution. A number of industrial and agricultural processes introduce substances in water supplies that reduce their quality and can render them unsafe for consumption. Examples include the production of plastics and pesticides.
Outline the principle of the ‘water footprint’ evaluation process, and illustrate your explanation using an example for a product or process. Include direct and indirect water use and relevant sources.
A water footprint assessment is a process that quantifies a water footprint, assesses it in relation to sustainability, efficiency, and equitability of water use, with the ultimate aim of identifying strategic actions that must be prioritised in order to increase the sustainability of the footprint. For example. the water footprint of the UK is comprised of direct water use through drinking, cleaning, washing, and the indirect water use which is a combination of water used in the UK to produce goods used here, and the water used in other countries to produced goods used here.
Example:
Water footprint of a takeaway cup of coffee.
Direct water use: - Hot water, supplied from tap Indirect water use: - Production of sugar - Production of coffee beans - Production of milk - Production of plastic lid - Production of cup
Sustainability consideration:
Crops fed by rainwater or groundwater?
Sugar beet, sugar cane, artificial sweeteners. Cane sugar generally has larger water footprint, may be from water stressed India? Or could be from Brazil.
Cows milk, could use vegan alternatives?
Production of coffee beans from water stressed such as Ethiopia or Brazil which has lots of water? Should pay more if water stressed to help them combat this problem, without having to negatively impact them economically.
Production of plastics: what is the source of water used for manufacturing, could install recycling options, for cooling etc.
Membrane technology has matured over the past 20 years and now offers several viable applications around water treatment. For a membrane-based water treatment technology, consider its role in helping to conserve/reuse our valuable water resource and address the following issues:
- what is the basic process and what does it do?
- comment on the type of membrane used and the likely operating conditions
- what happens to the pollutants as they pass through the membrane process, what is their end fate?
Draw flowsheet first
- Membrane-based water treatment used at the O2 arena. Used to treat water from three sources, and make it suitable for re-use as water for toilet flushing. Rainwater is collected from the roof, grey water is collected from the washbasins, and the poor quality groundwater from the surrounding area is collected. Membranes are integrated with biological filtration, chemical oxidation and reed beds. Source quality issues: Grey water: BODs/surfactants (from soap) Debris/solids TDS (total dissolved solids) Rain water: BOD (from bird poo) Debris/solids (from roof) Borehole water: salinity/TDS dissolved H2S + Fe, ammonia and aromatic hydrocarbons from fuel from ships/boats hardness
Purpose for UF: to prevent fouling of reverse osmosis membrane, purpose for RO: to completely disinfect due to public safety concerns. And also remove things so pipes don’t corrode.
- First, a UF membrane is used, likely operating condition is 3-80 psi, 0.01-0.1 μm then a reverse osmosis membrane is used, likely operating pressure is 800-1200 psi, <0.0001 μm.
- Reed beds take up nitrogen and phosphorus naturally as nutrients.
H2O2 oxidises ammonia, H2S, hydrocarbons and the granular activated carbon then allows for absorption.
Biological aerated filter oxidises pollutants, gets rid of BODs/surfactants and bacteria, whilst solids are removed by the filter. Waste goes to sewage.
All of this is combined and goes through UF prior to RO to make sure RO does not foul. RO disinfects - guarantees the removal of any biological activity.
Total 72% recovery
Conventional wastewater treatment plant flowsheet
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Explain the purpose and function of each of the units in the WWTP sequence. Indicate what is happening to the major pollutant groups at each stage, including any reactions, and what is coming in and out of each of the process stages.
Primary treatment:
In: sewage influent
A bar screen is a mechanical filter used to remove large objects, such as rags and plastics, from wastewater. Separation of storm water, removal or sand and grit, removal of oil or grease if present in large amounts and removal or disintegration of gross solids. Out: large solids, grit transported away.
Primary Treatment:
Removes settle-able solids through sedimentation or filtration. Solids are separated off as sludge that then goes to the sludge holding tank, after which it will be treated. The water stream moves on to the secondary treatment.
Secondary Treatment: Dissolved or colloidal organics are oxidised by micro-organisms. Products are carbon dioxide, water and biomass. Typical options are trickling biological filters or the activated sludge process. Activated sludge process: aerobic biological oxidation of organic material in wastewater. The basic activated sludge process comprises of a reactor in which the microorganisms responsible for the treatment are kept in suspension and aerated. In addition, liquid-solid separation takes place downstream of the reactor and a recycle stream returns microbial solids to the reactor -> this from clarifier? Some of the solids collected in the secondary clarifier (return activated sludge) are sent back to the aeration tank to treat more wastewater and the excess (waste activated sludge) is pumped to another location in the plant for further treatment. The clean water that flows out the top of the clarifier is sent along for disinfection. Reactions: BOD oxidation and new cell synthesis Endogenous respiration [EXPLAIN REACTIONS]
Tertiary Treatment:
Further polishing to remove residual BOD. Chorine disinfects. May also include nitrogen and phosphorus removal. In wastewater treatment, sodium bisulfite is often added following disinfection with chlorine prior to discharging the effluent to the receiving water. Residual chlorine can have a negative impact on aquatic life.
Sludge treatment:
The anaerobic digester converts about 40-60% of the organic solids to methane and CO2, methane can then be used for power. The residual organic matter is chemically stable, nearly odourless and contains significantly reduced levels of pathogens. Dewatering: the purpose of sludge thickening is to reduce its volume, typical sludge comes at 1% solid fraction e.g thickening to 2% halves its volume. Sludge after anaerobic digestion and drying can be used as a fertilizer.
The conventional wastewater treatment process can be upgraded to allow water recycling rather than water release to the environment. Explain how this upgrade could be implemented and how the additional/alternative process works.
Water can be upgraded by using a membrane to ensure the complete removal of all biological activity, as well as being able to remove salination, water hardness. Following biological treatment, the water can be passed to a UF membrane, followed by RO and then used for agricultural irrigation or industrial processes. Example: O2 Arena
As an additional source of income, water companies will often accept liquid wastes from industrial sources. Some of these wastes however could be harmful to the bacteria in the waste water treatment process, so a pre-treatment step is required. Give three examples of such wastes and indicate the appropriate pre-treatment method.
Organic compounds can come from pharma industry. Can be treated using oxidation and adsorption.
Heavy metals can be treated using oxidation or reduction, chemical precipitation, filtration. Mining industry, using acid drainage systems can disperse the acid solution left after the drainage process in groundwater.
Ammonia, can be treated using air stripping or recovery. Agricultural industry from fertilisers.
Describe the primary purpose of each component or feature of the membrane bioreactor.
Air stream: supplies oxygen for respiration to the aerobic microorganisms required for the biological processing, and keeps the microbial aggregates continually suspended, ensuring maximum contact with the wastewater.
Biological process area: dissolved or colloidal organisms are oxidised by micro-organisms producing CO2, water and biomass, locking them into solid structures that can more easily be removed. Carbon may also be removed from the water through endogenous respiration.
Membrane filtration process:
results in high effluent quality as the membrane disinfect the wastewater through selectively not allowing pathogens etc into the permeate. Separates solids such as the biomass produced, taking away the need for the secondary and tertiary clarifiers and filtration techniques required for the conventional activated sludge process.
Comment on the fate of the components in the waste water and explain the biochemical reactions which are responsible for breaking down the organic pollutants. What are the products of these reactions? Where do they end up?
- Biochemical reactions
Locked in into biomass, then leave reactor as waste sludge to be processed. - Nitrogen reactions?
Describe how the integration of a membrane system with the bioreactor provides the opportunity to have greater performance in terms of waste water treatment compared to more conventional bioreactor types such as the activated sludge process.
Removes the need for clarifiers, air stripping, sand filtration, etc so less time.
Capable of simultaneous biological treatment, and disinfection of the effluent resulting in treated water being of high quality.
Complete separation of hydraulic retention time (HRT) and suspended solids retention time (SRT) which provides optimum control of biological reactions and greater reliability and flexibility in use. Complete control of sludge age is important to allow development of slow-growing microorganisms such as nitrifying bacteria.
Ability to treat high strength waste - new market potential.
Ability to deal with variations and fluctuations in both the hydraulic load (m3 m-3 h-1) and organic load (kg BOD m-3 h-1) to the system.
Reduced sludge produced compared with other aerobic processes.
Pollutants effects classification
Having a significant toxicity on humans.
Having a significant toxicity on the aquatic environment.
Having a significant toxicity on the ecosystem.
Having a significant effect on materials and structures used in our infrastructure i.e acid rain.
Having a significant persistence in soil or a tendency to bio-accumulate -> things getting into food chain.
Being likely to occur in significant concentrations at many sites.
What are:
- carcinogenic
- mutagenic
- teratogenic
- potential to cause cancer
- potential to cause genetic mutations, altering DNA
- potential to affect the development of an embryo
What is BOD?
Biochemical oxygen demand: Indicator of the polluting capacity of an effluent where pollution is caused by the take-up of dissolved oxygen by microorganisms that decompose the organic material present in the effluent. Measured as the weight (mg) of oxygen used by 1L of sample effluent stored in darkness for 5 days.
What is COD?
Indicator of water effluent quality, which measures oxygen demand by chemical (as distinct from biological means) using potassium dichromate as the oxidising agent.
EU water framework directive 2000
Legislates for the composition of a water stream that is discharged after treatment on these basis particular limits and regulations:
- Soluble biodegradable organics:
Organics that might escape from biological treatment
Organics formed as metabolic intermediates during biological treatment
cellular components
- Suspended organic matter
biological solids that pass through the treatment plant
colloidal organics that pass through the treatment plant
- Non-biodegradable organics
May be originally present in the wastewater
May result as a by-product from biological treatment
- Nutrients:
Nitrogen, EU nitrates directive 1991 and NVZs
Phosphorus, new legislation is currently being developed.
