The most popular wastewater treatment is Oxidation ponds, which will produce an effluent meeting the recommended microbiological and chemical quality guidelines both at low cost and with minimum operational and maintenance cost. A low level of treatment is especially suitable in developing countries, not only from the point of view of cost but also in terms of the difficulty of operating complex systems. In many locations it will be better to design the reuse system to accept a low-grade of effluent rather than to rely on advanced treatment processes producing a reclaimed effluent which continuously meets a stringent quality standard.
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Oxidation ponds are now regarded as the method of first choice for the treatment of wastewater in many parts of the world. In Europe, for example Oxidation ponds are very widely used for small rural communities (approximately upto 2000 population but larger systems exist in Mediterranean France and also in Spain and Portugal). In the United States one third of all wastewater treatment plants are Oxidation ponds, usually serving populations up to 5000. However, in warmer climates (the Middle East, Africa, Asia and Latin America) ponds are commonly used for larger populations (upto around 1 million). In developing countries and especially in the tropical and equatorial regions sewage treatment by Oxidation ponds has been considered an ideal way of using natural processes to improve sewage effluents.
Oxidation ponds, also called Waste Stabilization Ponds (WSP) or lagoons, are holding basins used for secondary wastewater (sewage effluents) treatment where decomposition of organic matter is processed naturally, i.e. biologically. The activity in the Oxidation ponds is a complex symbiosis of bacteria and Algae, which stabilizes the waste and reduces pathogens. The result of this biological process is to convert the organic content of the effluent to more stable and less offensive forms. Oxidation ponds are used to treat a variety of wastewaters, from domestic wastewaters to complex industrial waters, and they function under a wide range of weather conditions, i.e. tropical to arctic. They can be used alone or in combination with treatment processes.
There are normally at least two ponds constructed. The first pond reduces the organic material using aerobic digestion while the second pond polishes the effluent and reduces the pathogens present in sewage. Sewage enters a large pond after passing through a settling and screening chamber. After retention for several days, the flow is often passed into a second pond for further treatment before it is discharged into a drain. Bacteria already present in sewage acts to break down organic matter using oxygen from the surface of the pond. Oxidation ponds need to be dislodged periodically in order to work effectively.
There are three types of Oxidation Ponds which are,
Usually an Oxidation ponds system comprises a single series of the three ponds types. In essence, anaerobic and facultative ponds are designed for BOD removal (Biological Oxidation Demand) and maturation ponds for pathogen removal, although some BOD removal occurs in maturation ponds and some pathogen removal in anaerobic and facultative ponds. In many instances only anaerobic and facultative ponds are required.
Anaerobic ponds are deep treatment ponds that exclude oxygen and encourage the growth of bacteria, which breaks down the effluent. It's in the anaerobic pond that the effluent begins breaking down in the absence of oxygen "anaerobically". The anaerobic pond acts like an uncovered septic tank. Anaerobic bacteria break down the organic matter in the effluent, releasing methane and carbon dioxide. Sludge is deposited on the bottom and a crust forms on the surface as .
Anaerobic ponds are commonly 2-5 m deep and receive such a high Organic loading (usually > 100 g BOD/m3 d equivalent to > 3000 kg/ha/d for a depth of 3 m). They contain an Organic loading that is very high relative to the amount of Oxygen entering the pond, which maintains anaerobic conditions to the pond surface. Anaerobic ponds don't contain algae, although occasionally a thin film of mainly Chlamydomonas can be seen at the surface. They work extremely well in warm climate (can attain 60-85% BOD removal) and have relatively short retention time (for BOD of up to 300 mg/l, one day is sufficient at temperature > 20oC).
Facultative ponds (1-2 m deep) are of two types: primary facultative ponds, which receive raw wastewater and secondary facultative ponds, which receive settled wastewater (usually the effluent from anaerobic ponds). They are designed for BOD removal on the basis of a relatively low surface loading (100-400 kg BOD/ha d at temperature between 20°C and 25°C) to permit the development of a healthy Algal population as the Oxygen for BOD removal by the pond bacteria is mostly generated by Algal photosynthesis. Due to the Algae facultative ponds are coloured dark green, although they may occasionally appear red or pink (especially when slightly overloaded) due to the presence of Anaerobic purple sulphide-oxidizing photosynthetic bacteria.
Algae populations within the aerobic pond require sunlight. They develop and produce oxygen in excess of their own requirements. It is this excess of oxygen that is used by bacteria to further break down the Organic matter within the effluent. The algal production of oxygen occurs near the surface of aerobic ponds to the depth to which light can penetrate. Oxygen can also be introduced by wind.
This facultative condition occurs because high oxygen levels cannot be maintained to the total depth of aerobic ponds. So, a fully aerobic surface layer develops along with an aerobic/anaerobic intermediate layer, and a fully anaerobic layer on the pond bottom. Oxygen is unable to be maintained at the lower layers when, the pond is too deep and the colour is too dark to allow light to penetrate fully.
- The demand for oxygen in the lower layer is higher than the supply. Demand is increased with high levels of organic matter. The anaerobic layer will be deeper in an aerobic pond where there is an extremely high organic matter content of the inflowing effluent.
- The surface layer, rich in oxygen is not adequately mixed with the bottom layer.
These ponds receive the effluent from a facultative pond and its size and number depends on the required bacteriological quality of the final effluent. Maturation ponds are shallow (1.0-1.5 m) m, and their entire volume is well oxygenated throughout the day. Their algal population is much more diverse than of facultative ponds. Thus, the algal diversity increases from pond to pond along the series. The main removal mechanisms especially of pathogens and faecal coliforms are ruled by algal activity in synergy with photo-oxidation.
On the other hand, maturation ponds only achieve a small removal of BOD5, but their contribution to nitrogen and phosphorus removal is more significant. A total nitrogen removal of 80% in all waste Oxodation pond systems, which in this figure corresponds to 95% ammonia removal. It should be emphasised that most ammonia and nitrogen is removed in maturation ponds. However, the total phosphorus removal in maturation ponds system is low, usually less than 50%.
Methodology And Its Evaluation
As part of my report, I choose Waiwera Oxidation pond in Rodney District council. This pond has been designed for a population of about 1920 with the Orewa ponds. Septic tank sludge is not permitted in this ponds, there are two ponds operated in Waiwera, which are medium depth of 1.7m. This pond has flat clay bottoms and clay compacted with concrete wave barriers with a 25 degrees slope (1 in 2.1). Water is transferred between ponds through a submerged pipe and effluent is discharged to the Waiwera River with a submerged outlet. Council doesn't allow to discharge between 15th of December to 1st of February every year. So, the discharge outlet is shown in figure 4 and 5 below.
Discharged Treated Water To Waiwera River
These ponds are build on mangrove flats of the Waiwera River. The ponds have been operating since 1974 and the only problem occurred due to moribund blue-green algae. The ponds are operated as a primary pond, the smaller pond was used with estimated loading of only 36 kg BOD/ha day until 22nd December 1977, but when the larger pond became the primary pond the load then being approximately 21 kg BOD/ha day.
In Waiwera Oxidation pond blue-green algae were Microcystis, aeruginosa and Anabaenopsis. In pond 1 green algae were only dominant during August to September 1977, when Chlorella was the most number in species, but during march to April 1978 when Actinastrum, hantzschii was the only dominant. While changing from pond 1 to the primary pond didn't appeared to affect the algal species. Which can be seen in figure 6 down below.
Blue-Green Algae In Pond
Green algae were dominant for more time in pond 2 rather than pond 1. During that time pond 2 was the primary pond. Selesnastrum minutum was a dominant for a short period in July 1977, then Chlorella and Micractinium pusillum then Actinastrum hantzschii during mid-November 1977 till mid-January 1978.
Faecal coliform bacteria were in higher number in winter period, when the lower than usual removal in the pond. The remaining time removal rate was in excess of 90%, the counting ranged from 9 to 4300 MPN/100ml. The highest consistent removal rate was achieved between January and March 1978, when the detention time was highest.
As seen in the above table 1, Oxidation pond water temperatures ranged from 10.4⁰C to 26.0⁰C. Dissolved Oxygen(D.O) concentrations in both ponds were usually close or over the saturation value except in low algal numbers. In tertiary pond, pH values varied throughout the year, being less than 9 units in the autumn and winter, and exceeding 10 units during times of high blue-green algal numbers in the spring and summer, the highest value recorded being 10.5 units. The chloride concentration in the Waiwera ponds was exceptionally high as a result of thermal water infiltration. The concentration fell each winter through dilution with storm water, the maximum value was 776 g/m3 in January 1978, and the concentration did not fall below 700 g/m3 until May. Biological Oxygen Demand (BOD) concentration varied, being less than 20 g/m3 for most of the year, rising to higher values particularly after Algal blooms. Inorganic-N concentrations were noticeably lower than in most other pond systems, the ammonium-N concentration usually being around 1 g/m3 and the nitrate-N concentration, highest during winter, only being above 0.5 g/m3 in June/July 1977. The total non-filterable residue concentration was highest when dense blooms of Microcystis aeruginosa were present. The turbidity followed the algal count closely.
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Mineral water infiltration into the Waiwera ponds was implicated by the high lithium concentration being at least 25 times greater than in ponds not influenced by thermal waters, and 2.5 times greater than that in seawater. The high Na:K ratio of 38:1 also favours the thermal water (ratio 9:1). The sodium and chloride concentrations contributed by the thermal water (Na:635 g/m3; c1: g/m3) would be 203 g/m3 and 330 g/m3 respectively, leaving a difference of 75 g/m3 Na and 77 g/m3 C1 which is within the range expected from domestic sewage.
Following in the Table 2 and Table 3 and graph 1 shows the Dominant Algae in pond 1 and pond 2 in 1977 and 1978.
Order Of Dominant Algae In Waiwera Pond 1 In 1977
Scan And Paste It
Order Of Dominant Algae In Waiwera Pond 2 In 1977
Scan And Paste It
Efficiency depending on the loading rate, temperature, BOD concentration, engineering details of the pond, and maintenance of the pond, particularly with respect to dislodging and crust control.
So, the efficiency is quiet effective in that case proved by the above data.
There are some advantages and disadvantages of the Oxidation ponds for smaller population area:
- Oxidation ponds will produce an effluent meeting, the recommended microbiological and chemical quality guidelines both at low cost with minimal operational and maintenance requirements.
- It costs low a level of treatment as possible is especially desirable in developing countries, not only from the point of view of cost but also in the difficulty of operating complex systems reliably.
- It makes the environment better as well as cleaning up the water.
- It deals a large amount of wastewater at same time.
- It utilizes the natural resource to help human beings and it can be used in low population.
- Oxidation ponds in some circumstances create insect and odours problems. But their main disadvantage for a small beach or lakeside community is the relatively large areas of flat land required conveniently located nearby.
- Impervious subsoil's below the pond are necessary to prevent excessive loss by infiltration to the ground water, failing this an impermeable membrane must be laid to keep effluent from escaping through the base and sides of the pond.
- Lastly, the depth of the pond is not advised to be increased above 2.0 m as efficiency drops away which is more and it occupies large area.
The purpose of this report is to give a clear concept of the Oxidation Pond and its BOD level and Algae. As well as, the report provides a number of data of a real case in different year.
- Oxidation ponds require larger space and useful for developing countries and rural areas, where there is low population around 2000 - 3000.
- It is low capital treatment plant which is more economic.
- Oxidation pond treatment plants are considered useful because of their low capital costs, their easy to maintenance and their potentially longer life-cycles.
- Oxidation ponds proved to be one of the most efficient, high performance and low-cost Waste Water Treatment Technology used around the world.
- Oxidation pond water temperatures ranged from 10.4⁰C to 26.0⁰C during that time.
There are many aspects of risk, so still need to take perfect steps while constructing the Oxidation pond or Stabilization ponds. They are pond embankment breach, erosion, flooding, noise, insect attraction etc. It would be better to take consideration to those elements when it is designed.
The data for this research was provided by Rodney District Council, New Zealand. The assistance of the staff involved, to arrange time to visit the site. And thanks to Glenys Rule, who took me the site visit, gave me some site maps and photos and the excellent advice.
Special thanks to Babar Mahmood (Course coordinator of this course, Senior Lecturer, Programme Director for BE and Programme Coordinator for BEngTech (Civil) Programmes) for giving me this chance to learn about the Oxidation pond.
Abis, Karen L. (2002). The performance of facultative waste stabilization
ponds in the United Kingdom. Ph.D. thesis, U.K. University of Leeds.
“Loan Proposal for Sewage Treatment and Disposal for Waiwera,” Waitemata County Council
“A survey of Oxidation Ponds in Auckland Region,” Auckland Regional Authority, works division 1979
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