2.0 General description of oxidation pond functions, processes, and performance
An overview of wastewater oxidation ponds in general is presented at this stage to provide a basis for discussion of the function and performance of dairy shed wastewater treatment ponds.
First the concept of Biological Oxidation Demand (BOD) should be introduced. Organic compounds in wastewater may be used as food for bacteria which can biochemically digest or oxidise the organic compounds to produce energy for growth. This oxidation of organic material, if done under aerobic conditions (i.e. in the presence of oxygen), "consumes" oxygen and produces carbon dioxide. An organic waste can therefore be said to have a biochemical oxygen demand, i.e. the amount of oxygen required by aerobic bacteria to oxidise it.
The term BOD is used to refer to the organic material in a waste and can be used in quantitative expressions relating to organic material. e.g. the expression g BOD or kg BOD describes an amount of organic material. The amount of BOD in a specific volume of wastewater is the concentration or strength of the wastewater and is expressed in terms such as g/m3 or mg/L or parts per million of BOD (all numerically equivalent). The loading rate of organic waste to a treatment system or a receiving environment (e.g. land) is expressed as a mass of BOD/ volume (or area) of treatment system per unit of time: e.g- g BOD/m3/day for loading rate of an anaerobic pond; g BOD/m2/day to a facultative pond or to land.
BOD is measured in a five-day test of oxygen consumption. The BOD value derived from this test is usually expressed as the BOD5 of the wastewater.
Small ponds that receive a reasonably high input of plant nutrients generally develop ecosystems that feature algal populations that produce oxygen in excess of the respiration requirements of the algae. This "excess" oxygen can he used by bacteria to oxidise biodegradable organic matter (quantified as BOD5) entering the pond.
This principle forms the basis of natural-aeration waste oxidation ponds, wherein bacterial degradation of organic waste provides carbon dioxide and nutrients to sustain algal photosynthesis and production of oxygen that the bacteria then use.
Oxidation ponds are an effective, low-cost, and simple technology for reducing the BOD of a wastewater before it is discharged to an aquatic ecosystem (e.g., a cleanwater stream). Excessive BOD loading to a stream may cause a depression in dissolved oxygen concentrations to the point of causing stress to some stream life, or may cause growth of stream-bottom bacteria to the extent of nuisance accumulation of bacterial biomass.
In natural-aeration oxidation ponds, algal production of oxygen occurs near the surface of ponds, in a "euphotic" zone limited by the depth of light penetration into the pond. A small amount of oxygen also enters the pond by surface diffusion. If the depth of a pond is greater than the euphotic zone, and wind mixing does not mix the near-surface oxygenated water the full depth of the pond, or if the amount of oxygen produced or transferred to deeper water is less than is needed by bacterial respiration there, then an anaerobic (no oxygen) zone develops at the bottom of the pond. How far the anaerobic zone extends toward the pond surface depends on the balance of oxygen supply and oxygen demand. Oxygen demand in a pond depends on the loading of biodegradable organic waste to the pond.
The concepts outlined above are used in the design of oxidation ponds. There are fundamentally three types of pond, depending on the dissolved oxygen profile in the pond. At one extreme are shallow (c. 0.5 m deep), fully aerobic ponds, where a euphotic zone that reaches the bottom and effective mixing cause dissolved oxygen to be present the full depth of the pond. Although the BOD5 of the influent waste is very efficiently reduced, the concentration of algal and bacterial suspended solids in the effluent can be high resulting in a high BOD5 concentration in the effluent.
At the other extreme are anaerobic ponds, where organic loading is so high relative to oxygen entering the pond that the pond is anaerobic right to the surface. Under these conditions, fermentation processes and anaerobic oxidation can remove about 70 percent of the BOD5 of the waste. Anaerobic ponds are mixed to some extent by the bubbling of gases (carbon dioxide and methane) produced in the pond. The suspended solids turbidity caused by mixing reduces light penetration, and algal production of oxygen, when it occurs at all in these ponds, is negligible.
Anaerobic ponds are a very cost-effective way of reducing the BOD5 of medium to high strength wastes. Anaerobic ponds can be deep and thereby economical in terms of land use, and significant BOD5 reduction is achieved without the need of energy input for treatment.
A third tyre of oxidation pond is the "facultative pond". The depth of natural-aeration facultative ponds - usually 1.0 to 1.5m - is too deep for oxygen to penetrate to the bottom of the pond, and an anaerobic zone develops there. Solids from the incoming waste settle into the anaerobic sludge near the bottom of the pond and degrade anaerobically releasing soluble degradable organic material and nutrients which diffuse upwards in the pond.
Near the top of the pond oxygen is supplied by algal photosynthesis and to a limited extent by diffusion from the air. There is dissolved oxygen present to only a few centimetres depth at night, but dissolved oxygen diffuses deeper during daylight. Thus there exists a fully aerobic zone at the top of the pond, and between this and the anaerobic zone at the bottom there is a middle zone where oxygen is cyclically present and bacterial respiration is "facultatively" aerobic-anaerobic.
A facultative oxidation pond receiving sewage typically achieves between 70 to 95 percent removal of BOD5 (non-filtered) at a loading rate to the pond of from 2.2 to 3.5g BOD5/m2/day depending on temperature. An effluent quality standard of 30g BOD5/m3 is typically set. The BOD5 in the pond effluent comprises both residual BOD5 from the influent waste and new BOD5 created in the pond in the form of algae. Algae may form the major part of the BOD5 in the discharge from a natural-aeration aerobic pond.
Facultative oxidation ponds arc directed at reduction of BOD5 and to a lesser extent suspended solids in wastewater. Pathogen reduction is not targeted in the design of facultative oxidation ponds. Where significant pathogen reduction by ponding is desired, a series of small "maturation" ponds is normally used after the oxidation pond(s).
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