ECOSAN C2_Assigment
Rob D’hondt
Questions C2U1: Treatment aspects for urine, faeces and greywater
1. List the four main groups of pathogens that may be present in human excreta and name for
each group a disease that it causes.
- Bacteria: Typhoid
- (parasitic) Protozoa: Giardiasis
- Helminthes: Schistosomiasis
- Viruses: Hepatitis
2. Which 3 processes play an important role in pathogen kill by storage of urine?
- pH increase from conversion of urea to ammonia
- Increase of ammonia concentration
- Time
3. What are the minimum rule-of-thumb threshold values for ammonia concentration (mM) and
storage temperature (°C) below which safe unrestricted reuse of urine becomes difficult?
The minimum thresholds for safe unrestricted reuse of urine are 40 mM NH3 and a storage
temperature of 20°C.
4. Name 3 arguments promoting and 3 questioning the need of urine sanitization before reuse.
Urine sanitization is needed, because:
- Urine can be contaminated by bacteria in the urine tract
- Urine can be contaminated by faecal contamination
- People infected with certain diseases can spread these through urine (schistosomiasis, renal
tuberculosis)
Urine sanitization is not needed, because:
- Urine in general contains few pathogens
- Urine-oral transmission of diseases is much less likely that faecal-oral transmission
- A multiple-barrier approach should always be adopted for the reuse of urine
5. Name 2 advantages and 2 disadvantages of using stored urine in reuse compared to ‘fresh’
urine.
Advantages:
- Pathogen kill is achieved
- Better timing of urine application is possible
Disadvantages:
- Storage costs
- Some possible nitrogen loss due to ammonia volatilization
6. Name 3 alternative low-tech options for urine management if agricultural reuse is not
possible.
1
ECOSAN C2_Assigment
-
Rob D’hondt
Discharge to sewer and wastewater treatment plant
Composting
Infiltration into the ground
7. Give 5 variables that promote pathogen die-off in faeces and give for each a technology
example.
- Storage time (storage)
- Temperature (thermophilic composting)
- pH (alkaline treatment, adding ash)
- Dryness (desiccation, adding ash)
- Solar light (spreading in the open)
8. Give 4 benefits of adding ash to faeces as a primary treatment step in a UDD toilet.
- Elevation of the pH, resulting in pathogen die-off
- Reduction of smell
- Covering of material, resulting in reduced fly-breeding and and improved aesthetics
- Reduction of moisture content, resulting in pathogen die-off
9. What is the recommended storage time of dry excreta at <20°C ambient temperature and at
>20°C?
1.5-2 years for less than 20°C, 1 year for warmer conditions.
10. Name 3 alternative low-tech options for faeces management if agricultural reuse is not
possible.
- Discharge to sewer and wastewater treatment plant
- Take to sanitary landfill (large scale)
- Dump in shallow holes and plant trees (e.g. Arborloo, small scale)
11. What makes separate greywater treatment preferable over treatment of conventional (mixed)
wastewater?
Greywater is easier to treat separately because it contains almost no pathogens and little
ammonia nitrogen.
12. Name the main purpose of the primary, secondary and tertiary greywater treatment steps.
- Primary: settling of suspended solids and large particles
- Secondary: removal of BOD
- Tertiary: disinfection and polishing/ removal of organic pollutants and heavy metals
13. Give 4 determining factors for the required level of greywater treatment.
- Population density
- Tolerable public health risk
- Receiving environment
2
ECOSAN C2_Assigment
-
Rob D’hondt
Type of reuse application
14. What is the strength of greywater and give a rule-of-thumb for describing high- and lowstrength greywater and the most appropriate treatment process for each.
The strength of greywater is the amount of organic matter (BOD) it contains. High strength
graywater contains >400mg BOD/l and is best treated with anaerobic processes (with
subsequent polishing). Low-strength greywater contains <400 mg BOD/l and is best treated with
aerobic processes.
15. Give 3 biological low-rate systems and 3 biological high-rate systems for the main greywater
treatment step.
Low-rate:
- Constructed wetlands
- Septic tanks
- Waste stabilization ponds
High-rate:
-
Trickling filters
Rotating biological contactors
Membrane bioreactors
16. Name 3 potential environmental and public health risks of greywater reuse in agriculture.
- Spreading of diseases
- Contamination of foods
- Damage to soil structure
Questions C2U5: Constructed wetlands
1. Give 3 possible applications of constructed wetlands in ecosan concepts.
- Greywater treatment
- Polishing (post-treatment) after anaerobic treatment of blackwater
- Faecal sludge treatment
2. Give 3 general strong points of constructed wetlands.
- Flexibility in site selection
- Simple operation and maintenance
- Aesthetical benefits
3. Give 4 general weak points of constructed wetlands.
- Design still largely empirical
3
ECOSAN C2_Assigment
-
Rob D’hondt
Start-up problems
Large space requirements compared to high-rate treatment systems
Mosquito problems in certain CW types with possible related malaria spreading
4. Give 4 selection criteria for selecting appropriate plant species for the CW and give 3 examples
of often used plant species.
- High biomass production
- Extensive root system
- Ability to withstand shock loads and short dry periods
- Preferably local, indigenous species
Example plant species: reeds (Phragmites), sedges (Cyperus papyrus), Duckweed (Lemna)
5. Give the 4 different types of CWs and compare their area requirements.
- Free water surface flow (FWS) CW: 5-10 m²/PE
- Vertical subsurface flow (VSSF) CW: 2-3 m²/PE
- Horizontal subsurface flow (HSSF) CW: 3-5 m²/PE
- Hybrid systems (a combination of the above): 2.5-3 m²/PE
6. Why is pre-treatment essential for subsurface flow CWs and what is an appropriate technique?
Settling of suspended solids and large particles is an important pretreatment step in subsurface
flow CWs to avoid blocking of the influent distribution systems and pore spaces (clogging). An
often used technique is settling tanks or septic tanks.
7. What is special about the redox conditions in CWs? Describe the different redox conditions
that prevail in the 4 CW types and that can exist in a specific system.
CW generally sustain a mosaic of different redox conditions, resulting in different removal
processes.
- FWS CW: aerobic in upper water layers, anaerobic in deeper layers and sludge
- HSSF CW: mostly anaerobic
- VSSF CW: most aerobic (intermittent loading)
- Hybrid: a combination of the above
Within a system:
-
Roots release oxygen, resulting in aerobic patches
Biofilms can be aerobic in the upper layers and anaerobic in the lower
8. What is the biofilm in CWs and where can it be found?
A biofilm is an aggregate of microorganisms attached to a surface. In CWs this surface are the
stems of plants (FWS) or the roots of plants and the filling medium (SSF).
4
ECOSAN C2_Assigment
9.
Rob D’hondt
Give 3 treatment mechanisms in CWs related to biofilms.
- BOD degradation
- Nitrogen removal through nitrification (aerobic) and denitrification (anaerobic)
- Pathogen die-off through predation
10. Give 3 pollutants that are removed in CW by adsorption in the soil, filling medium and organic
matter.
- Phosphorous (phosphate)
- Heavy metals
- Trace organics (micropollutants, pharmaceuticals)
11. Give 3 pollutants that are removed in CW by plant uptake.
- Nitrogen (ammonia and nitrate)
- Phosphorous (phosphate)
- Heavy metals
12. Which type of CW has the highest performance for nitrogen removal and why?
The hybrid type because it combines aerobic and anaerobic removal pathways.
13. Give 3 selection criteria for the filling medium of SSF CW.
- Hydraulic conductivity
- Local availability
- Phosphate sorption capacity
14. Give 3 advantages of harvesting plants from the CW and 3 advantages of NOT harvesting.
Advantages of harvesting:
- Net nutrient removal
- Reuse of plant material (building material, ornamental flowers)
- Avoiding pest breeding sites by preventing thick layers of plant material
Advantages of not harvesting:
-
Provision of organic layer for adsorption of pollutants
Provision of carbon source for denitrification
Sustaining the ecological functioning of the wetland
15. Give 4 possibilities to avoid mosquito breeding in FWS CW.
- Biological control agents e.g. certain fish species
- Support the growth of natural predators e.g. by providing open, unplanted water areas
- Reduce organic loading by pretreatment
- Temporary drying of the beds
5