Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2000-11-22
2001-12-11
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S616000, C210S621000
Reexamination Certificate
active
06328892
ABSTRACT:
The present invention relates to a liquid effluent treatment process and to a plant for treating a liquid effluent such as waste water.
U.S. Pat. No. 4,552,663 describes a process for the removal of ammoniacal nitrogen in the BOD sorption zone of a waste water treatment plant. The BOD sorption zone A, which may be provided with a blanket of nitrogen, comprises a tank partitioned to provide two or more liquid treating sections in order to approximate plug flow of the liquid through the BOD sorption zone A. Liquid from zone A is discharged into the BOD oxidation zone B and is aerated therein. Aeration in each liquid treating section of zone B is proposed using spargers for the purpose. From zone B the liquid passes to a sedimentation tank from which settled sludge is recycled to BOD sorption zone A.
In U.S. Pat. No. 3,953,327 it is proposed to treat sewage by anoxic denitrification while supplying methanol as food for bacteria, followed by stabilisation by aeration and sedimentation. Resulting sludge is recycled to the denitrification step and separated, relatively clear effluent is removed from the process.
A conventional Deep Shaft™ waste water treatment plant is described in GB-A-1473665. Such a plant includes a basin, a riser and a downcomer whose upper ends communicate with each other and with the basin and whose lower ends communicate with each other, as well as circulating means for circulating an effluent, such as sewage, around the system and means for supplying an oxygen-containing gas to the effluent in the downcomer. The downcomer is of a length such that the oxygen transfer rate into the effluent is at least 0.1 kg O
2
/hr/m
3
. At start up, air is injected into the riser causing its upper section to act as an air-lift pump. When the effluent is circulating at a suitable velocity, e.g. at least 1 meter/sec in the downcomer, the proportion of air which is supplied to the downcomer is increased. Eventually circulation can, if desired, be maintained by supplying air only to the downcomer. The point of injection of air into the downcomer and into the riser is preferably at a position between 0.1 and 0.4 times their total length below the level of the effluent (e.g. sewage) in the basin. Thus when the system extends from 150 to 250 meters below the level of effluent (e.g. sewage) in the basin, air is injected 15 to 100 meters below this level. It is further preferred that air injection takes place at a position more than 30 meters below the level of sewage or other effluent in the basin.
A further description of such a plant can be found in an article by B. Jones in Waste Management & the Environment, Vol 5(3), June 1995, pages 44 and 45.
The practical depth of the shaft in commercial plants, of which there are now more than 70 operating throughout the world, is from 50 to 150 meters. If air is injected into the downcomer 35 meters down a 100 meter shaft, then the path length for the air supplied to the downcomer is 165 meters, giving a contact time of around 3 minutes, compared with about 15 seconds in a conventional diffused air process. Most bubbles dissolve before reaching the bottom of the shaft. Because of the greater solubility of oxygen at the bottom of the shaft, due to the increased hydrostatic pressure, there is a large driving force for oxygen transfer. This has been shown to result in oxygen transfer figures of 3 kg O
2
/m
3
/hr compared to 0.05 to 0.2 kg O
2
/m
3
/hr for more conventional processes. This characteristic allows a Deep Shaft™ plant to treat strong wastes at an increased loading rate, expressed as the food/microorganism (f/m) ratio. Absorption efficiency of oxygen can reach 80% in a Deep Shaft™ plant compared with 15% in a diffused air process. These factors, plus the high turbulence of the liquid in the shaft, are the main cause of the high power efficiencies observed in the shaft (3 to 4 kg O
2
/kWhr compared to 1 kg O
2
/kWhr in a conventional process). Another advantage of the use of a Deep Shaft™ plant is that the high velocities (1 to 2 m/sec) ensure that solids do not settle out in the shaft, making primary settlement unnecessary.
Further details of Deep Shaft™ plants can be found in an article by D. A. Hines et al. in I. Chem. E. Symposium Series No. 41 at pages D1 to D10.
Bacteria in the liquid effluent circulating in the plant metabolise organic and certain inorganic molecules present in the effluent. Many types of effluent may be treated in this way. For example, domestic sewage, food waste, waste paper and industrial effluent from the textiles industry, the plastics industry and the steel industry are all treated currently by commercial plants of this type.
In operation, a conventional Deep Shaft™ plant is initially seeded with a load of activated sludge from a sewage plant. Once circulation of the effluent is initiated, the enhanced aeration of the system provides suitable conditions for the aerobic bacteria to thrive.
In the downcomer the bubbles of air entering the system tend to rise against the flow of the liquid effluent. However, once the liquid circulation rate reaches a value greater than the upward velocity of an air bubble, air bubbles entering the downcomer are drawn downwards by the moving effluent until, eventually, the air stream to the riser can be cut off, with air then being supplied to the system only in the downcomer. Because of the voidage in the effluent in the riser the specific gravity of the mixture of air and effluent in the riser can be as much as 0.20 g/cm
3
less than that of the unaerated liquid effluent at the top of the downcomer. Hence the circulation of effluent through the plant is assisted by the hydraulic pressure differential above the air supply point in the downcomer, due to the significant difference in specific gravity between the air bubble-containing aerated effluent in the riser and the non-aerated effluent at the top of the downcomer.
A conventional plant of this type is adequate for removal of molecules susceptible to metabolisation by aerobic bacteria. However, there are limitations on the effectiveness of such a plant in liquid effluent and waste water treatment. In particular, aerobic bacteria are not capable of digesting nitrates which may be produced in large quantities inside the shaft as a product of the metabolisation by the aerobic bacteria of ammonia. Ammonium ions are very common in many types of effluent, including domestic waste and sewage. Ammonia is also malodorous at high concentrations and toxic to aquatic organisms even in low concentrations; accordingly it is required to be removed from effluents by legislation in many countries. In addition it has a high biological oxygen demand (BOD). Although nitrates do not present the same problems as ammonia, their removal from waste water is highly desirable and is legislated for in some countries. Nitrates are natural fertilisers and their release into the environment, particularly in still waters, such as lakes and reservoirs, facilitates algal growth. Excessive nitrate release into environmental waters can give rise to a phenomenon known as eutrophication in which an aquatic environment becomes subject to excessive plant and algal growth and is eventually starved of important nutrients such as phosphorus because of intense algal bloom. Starvation and death of the algae population then follows and the decay of their cell material by aerobic metabolism depletes the water of oxygen, so that fish and other oxygen-dependent organisms are destroyed.
To combat the problem of nitrates in effluent which, when treated, is to be released into the environment, anoxic bacteria are conventionally used to reduce nitrates to nitrogen gas. These anoxic bacteria are provided in an anoxic tank upstream of the Deep Shaft™ equipment. A minor stream of effluent from the Deep Shaft™ equipment is generally recycled to the tank. However, such a system suffers from a number of disadvantages. In particular, the residence time of the recycled effluent in the anoxic tank is long enough for any aerobic bacterial activity in the recycle stream to be inhi
Barry Chester T.
Kvaerner Technology & Research Limited
Senniger Powers Leavitt & Roedel
LandOfFree
Liquid effluent treatment process and plant does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Liquid effluent treatment process and plant, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid effluent treatment process and plant will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2576163