Anaerobic ponds
- 1. Anaerobic Ponds Dr. Akepati S. Reddy School of Energy and Environment Thapar University, Patiala PUNJAB, INDIA
- 2. Anaerobic Ponds • Shallow, manmade anaerobic basins of waste stabilization pond systems (series of anaerobic, facultative and maturation ponds) • Used to treat domestic or municipal wastewater to – Remove and stabilize suspended solids – Remove biodegradable organic matter, BOD • Represent sustainable natural effluent treatment systems • Low cost, low energy, and low maintenance systems, and do not require skilled manpower – Construction involves earth moving, pond lining and pond embankment protection, and pond inlets and outlets – Operation and maintenance requirements are minimal (repair of embankments, cutting embankment grass, removing scum and vegetation, keeping both inlet and outlet clear, etc.) and requires only unskilled but carefully supervised labour – Cheapest and even land cost may not be acting against
- 4. Anaerobic Ponds • Anaerobic ponds represent primary treatment – sludge stabilization is add on feature – patogen removal (helminth eggs) is coincidental • Can be easily scaled down to small scale applications • Robust systems (withstand organic & hydraulic shocks and copes up well with heavy metals upto 60 mg/L) • Principal requirements are sufficient land, and soil with low coefficient of permeability (<10-7 m/sec.) • Suited to tropical and sub-tropical countries, like India – temp. (high throughout the year) is favourable – Inexpensive land, restricted foreign currency availability and shortage of skilled manpower favour the use Disadvantages • Requires more land (1-2 day HRT) • Potential odour and mosquito nuisance • Adverse environmental impacts (ground water pollution)
- 5. Preliminary Treatment Required Unless very small, the anaerobic pond must be preceeded by screening and grit removal facilities Provisions may be made for • Flow measurement and recording • Diverting the flow beyond 6 times to dry weather flow into storm water drain and/or receiving water course • Allowing a maximum of 3 times to dry weather flow into anaerobic ponds and diverting rest into facultative ponds • Bypassing the anaerobic pond
- 6. Anaerobic Ponds Small unmixed basins similar to uncovered septic tanks functioning to settle and stabilize particulate organic matter Very effective in removing heavy metals and in degrading organic compounds like phenols Mainly due to odour problems, often not used – primary facultative ponds or aerated lagoons or UASBs are preferred • Depth is 2-5 m (3 m typical) – ground conditions and local excavation costs actually influence the depth • Has sludge deposited at the bottom and scum layer at the top (scum layer can increase fly breeding!) • Single anaerobic pond is sufficient for wastewater with BOD5 <1000 mg/L – higher BOD5 requires a second pond in series • Typical TSS and BOD removals for domestic wastewater are 50-70% and 30-75% respectively
- 7. Anaerobic Ponds • Involves – Hydrolysis of particulate organic matter – Fermentative conversion of organic matter into VFA – Decomposition of VFA into acetic acid and H2 – Methanogenesis - very sensitive to VFA accumulation and associated pH drop • If sulfates & nitrates are present sulfate reduction and denitrification rather than methanogenesis will occur • Release biogas (methane and CO2) and even ammonia • Biogas can be recovered from covered anaerobic ponds - floating plastic membrane of three layers is usually used – Top high tensile UV-resistant geomembrane – Middle layer of polyfoam insulation and flotation – Base layer of high density polyethylene welded to the base 3242 8 3 4828 3 4824 3 24 dNHCO dban CH dban OH dba nNOHC dban
- 8. High strength, rapid VFA production and accumulation, and insufficient buffering capacity can prove problematic to the stabilization process Municipal sewage has high buffering capacity
- 9. Anaerobic Stabilization Process The stabilization process is influenced by • Temperature - works well in warmer climate (20-45C range, within which methane production increases by 7 fold with temp. increase by 5C • HRT (typical is 1 day and may depend on wastewater strength) • BOD loading rate (volumetric loading is used as the basis for design and 350 g/m3.day is taken as the upper limit) • pH - optimal range is 6-8 (optimal pH is 7) - very sensitive to pH <6.8 - adequate buffering capacity is very important • Sulfide – stabilization process produces sulfide from sulfate – Inhibitory to methanogens at 50-150 mg/L level - sulfide at 10- 12 mg/L level is lethal to vibrio cholerae (disinfection!) – Responsible for odour problems – Small amount of sulfide is beneficial –reacts with heavy metals and removes as metal sulfide precipitates
- 10. Anaerobic Stabilization Process The stabilization process is influenced by • Ammonia: toxic to the process (50% growth inhibition at 25- 30 mg/L level and very strong inhibition at >80 mg/L) – Free ammonia (which occurs at higher pH) is more toxic than ammonium ions • Toxic compounds - heavy metals and chloro-organics are inhibitory • Oxygen is also potentially toxic • Nutrients: • Degree of mixing:
- 11. Odour Problem • Main cause is sulfide (H2S) - caused by the release of H2S gas into atmosphere • H2S is present in the pond contents as H2S gas, as bisulfide ions and as sulfide ions • Relative concentrations are governed by pH (at 7.5 pH non- odourous bisulfide is predominant • Odour problem can be reduced by – Raising the pH to around 8 by adding lime – Recirculating oxygen rich effluent and forming aerobic top layer (oxidizes odorous sulfide) – Stimulating scum layer development (spread thin layer of straw) – Reducing organic loading rates (or increasing depth!) – Preventing short circuiting and avoiding dead zones • For properly designed pond, odour (due to H2S!) is not a problem if the sulfate in wastewater is <500 mg/L
- 12. Treatment Mechanisms Suspended solids • Sedimentation • Hydrolysis and stabilization Organic matter (or BOD) • Sedimentation (biodegradable VSS) and subsequent anaerobic digestion • 25-50% of the applied BOD may be released as methane • Bleeding of BOD back into the effluent can also occur Nutrients • Nitrogen removal (TKN) – Sedimentation of organic-N – Hydrolysis of organic-N into ammonical-N – Assimilation of ammonical-N and use as nutrient in biosynthesis – Release of ammonia into the atmosphere • Phosphorus removal • Sedimentation as both organic and inorganic phosphorus
- 13. Treatment Mechanisms Pathogen removal • Fecal coliform removal • Adsorption to particles and subsequent sedimentation (major contributor) • Natural decay or disinfection • Occurs by a combination of processes via complex interaction of various adverse environmental factors - starvation due to the lack of nutrients • Aquatic environment • Viruses – Apparently removed by adsorption on to settlable solids and consequent sedimentation • Helminth eggs & protozoan cysts – Removed by sedimentation – Most removal takes place anaerobic and facultative ponds
- 14. Design of Anaerobic Ponds Designed for BOD removal (removal of nutrients and pathogens is coincidental) The pond is sized on the basis of volumetric organic loading (can be 100-350 g/m3.day) – Temperature is the key design parameter - mean ambient air temperature of coldest month is used – For <10C 100 g/m3.day and 350 g/m3.day at ≥25C – Upper limit to the volumetric BOD loading is determined by odour emissions and minimum pH threshold value – optimum pH for methanogenesis is 6-8 – Loading should be >100 g/m3 for maintaining anaerobic conditions HRT is 1 to 3 days for municipal sewage (1 day for the sewage with <300 mg/L BOD5 at >20C)
- 15. Temp. T ( oC) Volumetric Loading (g/m3 d) BOD removal (%) <10 100 40 10-20 20T – 100 2T + 20 20-25 10T + 100 2T + 20 >25 350 70
- 16. Design of Anaerobic Ponds • Sludge accumulation occurs in the anaerobic pond, and decreases its HRT and necessitates timely desludging – When sludge occupies 1/3rd volume of the pond then desludging is usually required – Frequency of desludging can be estimated • Properly designed can achieve 40% BOD removal at <10C and >70% at ≥25C • Removal of fecal coliforms can be estimated by • Helminth eggs removal is fairly effective (upto 90%) – the removal is by plain sedimentation 2 0085.049.0exp41.01 R is HRT of anaerobic pond R is removal efficinecy anTB rw an K N N )(1 20 )( 19.16.2 T TBK is HRT in days of the anaerobic pond
- 17. Design of Anaerobic Ponds • Nitrogen in the treated effluent – Treated effluent contains mostly ammonical-N and small amount of organic-N (of the bacterial biomass leaving the pond as TSS) • Phosphorus in the treated effluent – Difficult to predict – Some may be lost as insoluble P into settled sludge – some may also be released from the settled sludge – A small fraction may be used in the anaerobic biosynthesis • Effluent TSS – Difficult to estimate – May depend on the outlet design, effluent turbulence level at the outlet zone, HRT, etc. – Suitably assumed in the light of the out let design, HRT and the local turbulence level
- 18. Physical Design Location/siting • The pond should be located >200 m downwind from the community and from the likely areas of future expansion – To discourage people from visiting the site – To give assurance to public against the unlikely odour problem • Should not be located within 2 km of airports (birds attracted to the ponds can constitute risk to air traffic) Geotechnical investigations of the site • Needed to ensure correct embankment design and to determine whether the pond requires lining or not • Includes – Determination of maximum height of the groundwater table – Collection of soil samples representing the soil profile up to a depth 1.0 m greater than the envisaged pond depth – Analysis of soil samples for particle size distribution; coefficient of permeability; maximum dry density and optimum moisture content; Atterberg limits; organic content, etc.
- 19. Physical Design: Embankments • Embankment design should allow vehicle access for maintenance • Better use the soil excavated from the site in the embankments construction – Organic soils and medium to coarse sands are not suitable • Compact the soil in 150-250 mm layers to 90% of its maximum dry density – coefficient of permeability should become <10-7 m/s • Ascertain slope stability (standard soil mechanics procedures for small earth dams can be used) – Plant slow-growing rhizomatous grass to increase the stability • Protect external embankments from storm water erosion (provide adequate drainage!) • Protect internal embankments from erosion by wave action - precast concrete slabs or stone rip-rap at TWL (Top water level)
- 20. Physical Design: Pond Lining • Seepage can be related to Coefficient of Permeability (k) as • When in situ k is >10-6 m/sec. then lining of ponds is needed – K <10-9 m/sec. indicates that the ponds seal naturally – K <10-9 m/sec. indicates no risk of ground water contamination • Portland cement (8 kg/m2) or plastic membranes or 150-300 mm layer of low permeability soil can be used as pond lining h l A Q k S .86400 k is coefficient of permeability (m/sec.) Qs is seepage loss (m3/day) A is pond area (m2) Δl is depth of soil above the aquifer (or more permeable stratum) in meters Δh is Δl plus pond water depth in meters
- 22. Physical Design: Pond Geometry • Usually rectangular with variable length to breadth ratio (L to B ratio) – can be gently curved if desired for aesthetic reasons – L to B ratio should be 2-3 to 1 – breadth is kept <24 m (imposed by excavators and desludging machinery) • Pond areas are estimated for mid water depth - constructor needs both pond bottom and top dimensions and depth • Volume can be related to TWL dimensions by • Pond liquid depth can be 2-5 m for anaerobic ponds sDWsDLsDWsDLLW D Va .42.2 6 Va is liquid volume of the pond L and W are top water level length and width D is depth and S is internal horizontal slope of embankment
- 23. Physical Design: Pond Geometry • The ponds should be provided with freeboard to prevent wind induced waves overtopping the embankment - Freeboard depends on the pond area – 0.5 m for ponds of <1 hectare area – 0.5 to 1.0 m for ponds of 1-3 hectares area – For ponds of >3 hectares area freeboard is calculated by • 2 or more parallel ponds are usually provided – Multiple systems need splitting of preliminary treated wastewater into equal parts (use weir penstocks) 1log 2 1 10 AF F is freeboard in meters A is pond area at TWL in m2
- 24. Physical Design: inlet and outlet structures • Relative position of inlet and outlet matters in minimizing hydraulic short-circuiting – Single inlet and single outlet can usually be sufficient – Locate the inlet and outlet away from the base of embankment – Locate them in the diagonally opposite corners of the pond • Inlets and outlets should be simple and inexpensive and should permit collection of pond samples with ease • Inlets should discharge well below the liquid level (to prevent scum disturbance and to minimize odour problems) • Protect outlets against scum discharge (provide scum guard) • Scum guard depth determines effluent take off level • Effluent take off should be below the surface crust and above the bottom sludge – recommended is 0.3 m Depth of flow over the outlet overflow weir is related to the weir loading by 2 3 0567.0 hq q is weir loading rate in L/m.sec. h is depth of flow in mm
- 26. Physical Design • Consider using baffles at the inlets and the outlets • To avoid short-circuiting • To avoid disturbing of the scum layer • To shield the outlet from scum entry • Bypass to anaerobic ponds is needed – facilitates desludging of anaerobic ponds • If needed make provisions for recirculating and mixing the final effluent with the influent after preliminary treatment – Needed to achieve odour control specially when the influent is septic (upto 50% may be recirculated) • Surround the ponds by a chain link fence and provide padlocked gates • Post warning notices indicating the hazards