Waste Stabilisation Ponds / Oxidation Ponds

Question 1

Is there still a place for WSP in todays waste management practices?

Answer 1

YES

• use of solar and wind energy
• excellent disinfection if short circuiting is minimised
• potential to use new technologies to improve effluent quality

Question 2

WSP

Answer 2

Waste Stabilisation Ponds

Question 3

Background of WSP

Answer 3

1. Many inefficient WSP systems
2. WSP are “old” technology
3. WSP can often be upgraded to higher performance at modest cost
4. Too little understanding of public health benefits
5. Economic and simple in comparison with in-tank systems
6. WSP are used worldwide

Question 4

Current WSP Design - “Best Practice”

Answer 4

• fine screening
• anaerobic or settlement zones for high solids wastes > 2m depth
• mechanical aerator/mixer assistance
• avoidance of localised overloading
• mixing essential in calm conditions to bring algae to surface for dose of sunlight
• biofilm surface on bank rock protection vs bulk volume requirements
• multiple ponds-in-series for greater efficiency
• to reduce short circuiting, position inlet and outlets with flow against prevailing wind
• flat floors reduce wind driven current short circuits
• stub baffles effective in smaller ponds but not effective in large 40ha ponds
• primary ponds can be nearly square to allow complete mixing and encourage algae growth
• design for 84kgBOD/ha.d, or 1200 persons/ha
• 2 or 3 primary ponds in parallel to give flexible loading if one pond is “upset”
• maturation ponds can be elongated to approximate plug flow. Design second pond to be 30% of primary pond area

Question 5

Improvement Options

Answer 5

• WSP variations
• rock filters or trickling filters for ammonia reduction
• wetlands

Question 6

WSP variations

Answer 6

• anaerobic/aerobic
• facultative ponds
• multi maturation cells in series
• AIPS
• high rate add-ons e.g. PETRO

Question 7

Add ons

Answer 7

• UV disinfection
• Chemical dosing
• Dissolved Air Flotation (DAF)
• Sand Filtration
• Membrane Filtration

Question 8

Indicator/Pathogen Reduction

Answer 8

• normally used indicators are all reduced by ponds in series
• concentrations

Question 9

Why does WSP have such good natural pathogen removal?

Answer 9

• sunlight
• dissolved oxygen
• temperature
• retention time
• pH
• predation
• settlement

Question 10

Sunlight Inactivation

Answer 10

• photobiological damage
• internal photo-oxidation
• external photo-oxidation
• adequate DO is important (>4 g/m3)

Question 11

Sludge Management

Answer 11

• removal every 15-30 years depending on storage depth
• trend of deeper ponds to allow for more sludge accumulation and greater depth of overlying liquid to control odours
• deeper pit near inlet creates ideal conditions for anaerobic digestion
• economic to construct an extra primary pond - allows a pond to be drained and sludge air-dried over summer

Question 12

WSP are sustainable because:

Answer 12

• utilise green energy
• sunlight provides excellent disinfection
• multiple ponds-in-series or baffled reduce short circuiting
• add-on processes can reduce ammonia, phosphorous and pathogens if needed
• sludge can be processed using solar and air drying, but extra space required