12. Waste Water and Sewage Treatement

Question 1

What are the problems associated with dumping sewage and waste water? 6

Answer 1

1. Organic compounds are readily oxidised by aerobic bacteria, leading to reduction in dissolved oxygen, leading to death of aquatic life
2. Solids cause silting of rivers ans esturies which require dredging. This interferes with hipping and wildlife
3. Toxins eg. ammonia and heavy metals from industry cause death to aquatic life and can build up in food chain
4. waste contains pathogenic bacteria and viruses, which pose a risk to human health
5. Bacteria levels rose very high in Irish sea due to dumping at blackpool, now some of cleanest beaches
6. Sewage treatments ensures waste disposal is safe

Question 2

What is the importance of water filtration? 4

Answer 2

1. Typhoid is water borne and so a good indicator of water quality
2. typhoid cases were extremely high in 1800s
3. dropped in 1906 when filtration began
4. Dropped even further when chlorination of drinking water began in 1913

Question 3

Describe the Roman sewage system. 6

Answer 3

1. romans had no knowledge of microbes but still knew sewage shouldn’t remain close to dweelings
2. realised it caused disease
3. Built toilets over running water eg streams, simple but effective
4. If no stream was available, would divert sewage away from area to river
5. York had the first underground sewage system, built by romans and still in use
6. After the romans left, a lot of the knowledge they brought faded, incl connection between sewage and ill heatlh

Question 4

Describe the elizabethan sewage system. 6

Answer 4

1. Channels carried raw sewage down the street
2. Upstairs of building overhangs downstairs to throw sewage straight into channels
3. channels went to river where water for drinking and washing was sourced
4. Indoor toilets were opposed, people didn’t want that in the house
5. 1855 - hot still summer and thames smell was so bad that parliament was abandoned for several months
6. Led to building of sewage system that is still used today, great engineering feat

Question 5

What is sewage treatment like in the UK today? 5

Answer 5

1. in 1981, 1x10^9 tonnes of water purified, this has increased due to population growth
2. equivalent to 100tonnes per person
3. was £150-200 per head, more like £350 now
4. Costs have risen due to EU legislation to increase environmental standards
5. Can no longer dump in oceans

Question 6

Describe severn trent water. 6

Answer 6

1. treats domestic waste of ver 8million ppl
2. 38500km of sewers
3. sewage is a thin liquid of less than 1% solids
4. solids include food, poo, detergents, fats, oils, greases, sand , clay and paper fibres
5. severn trent has 1048 sewage woks
6. smallest serves100 ppl, largest is birmingham and serves one million peeople

Question 7

Describe waste water treatment. 6

Answer 7

1. designed to reduce organic content, aerobic bacteria and prevent disease in dissolved o2
2. reduces suspended solids and pathogenic microbes
3. removes toxic NH4 by conversion to NO3
4. Microbes used to achieve these aims
5. Domestic waste only, specialised systems required for agricultural/industrial waste
6. exmaples include breweries, dairies, and slaughter houses

Question 8

How is waste concentration measured? 5

Answer 8

1. measure of success of the treatment
2. organic content defined by biological oxygen demand (BOD)
3. BOD is amount of oxygen consumed by aerobic bacteria in the dark over a set time
4. BOD is used to predict the effect of discharging waste and to determine effectiveness of purification process
5. By law, cannot exceed 20mgl-1 and be discharged

Question 9

Do the diagram for domestic sewage treatment.

Answer 9

DO IT

Question 10

What is primary screening and settling in terms of waste treatment? 3

Answer 10

1. Screening removes large solids by a mesh or filter
2. in primary settling tank, particles in primary sludge
3. remaining sludge rotated and removed with sludge sweep

Question 11

What is percolating filtration treatment. 4

Answer 11

1. This is an older method
2. water from primary tank is sprayed over bed of small stones
3. microbial processes occur here
4. the water filters through and it collected in the next tank

Question 12

What is the function of a percolating filter bed? 6

Answer 12

1. microbes form biofilm on the surface of clinker and those in top 0.5M oxidise organic compounds
2. below, when 02 becomes limiting, microbes oxidizes nh4 to no3 - nitrification
3. This reduces usable carbon and removes NH4
4. Regulation of waste spray is critical, if too high, biofilm gets too thick and causes blockages
5. Long term - balance of increasing biofilm due to growth, and removal by sloughing and grazing by invertebrates
6. percolating filters operational for 30-50yrs

Question 13

What is the activated sludge process? 6

Answer 13

1. newer process
2. differs from percolating filtration as microbes are not surface attached - no biofilms/rocks, but form clumps
3. Aerated by compressed air from air diffusers at base
4. Microbes grow as flocs and oxidise organic compounds and nh4
5. water spends 4-8 hours in tank, up to 10x faster than percolating filtration
6. however, running costs are higher and more sensitive to compositions of incoming water, toxins can kill significant proportions of bacteria, and biofilms are more resistant

Question 14

What is tertiary processing? 3

Answer 14

1. Sometimes referred to as polishing
2. consists of filtering through sand or pumping into an enclosed reed bed prior to release into rivers
3. Reeds planted by river by sewage companies

Question 15

What are anaerobic digesters? 6

Answer 15

1. used to treat sludge from aerobic treatment process
2. incubation in a sealed, anaerobic tank reactor at 35 degrees
3. Anaerobic as any residual o2 used very quickly by aerobes
4. anaerobes reduce organic matter by 50%
5. Organic matter -> ch3 + co2 + h2o
6. Excess methane gas piped to generator, where it can be burned or otherwise used to make electricity via stream

Question 16

What anaerobic process occurs during sewage treatment? 4

Answer 16

1. organic matter is converted to organic esters and acids, and alcohols and fatty acids by fermentatitve bacteria
2. These are then converted to acetic acid and H2 and CO2 by acetogenic bacteria
3. Must be careful, because too much h2 in a sealed tank can be explosively problematic
4. . These substances are converted to ch3 and co2 by methanogens

Question 17

What are the problems with anaerobic digestion in sewage treatment? 6

Answer 17

1. Three three classes of bacteria grow at different rates
2. methanogens double in 8-10hours, acetogens in 1-4hours and fermentative in 30 mins
3. So, fermentative bacteria create a lot of product and accetogen levels must be kept high enough to remove them
4. if methanogens can’t keep up, can lead to increase in H2, which leads to formation of long chain fatty acids/organic compounds
5. These are not used by methanogens, so processes must be monitored
6. If too many build up, they are toxic

Question 18

What waste products are produced during sewage treatment? 5

Answer 18

1. sludge has a low odour and is pathogen free
2. dried down to sludge cakes, supplied free to farmers as fertiliser but not city sludge due to high metal content
3. They can also be incinerated for electricity generation or dumped at sea (less frequent now)
4. ch3 and co2 can be used as fuel - biogas
5. can be used to heat anaerobic digester or, at larger plants, linked to the national grid

Question 19

How is sludge used as fertiliser? 3

Answer 19

1. Dried by air
2. Can be made into pellets
3. Viruses and pathogens are killed in anaerobic digestion

Question 20

What is the overall result of sewage treatment? 4

Answer 20

1. recued BOD by 95%
2. Seasonally dependent on rainfall
3. this has implications for each stage
4. water available is regulated by underground tanks, which divert floodwater into underground holding tanks to be released when there is no rain