GB2325926A

GB2325926A - Sewage treatment system having moving and fixed beds

Info

Publication number: GB2325926A
Application number: GB9811525A
Authority: GB
Inventors: Jonathan Edward Tul Strickland
Original assignee: AW Creative Technologies Ltd
Current assignee: AW Creative Technologies Ltd
Priority date: 1997-06-04
Filing date: 1998-05-28
Publication date: 1998-12-09
Also published as: GB9811525D0

Abstract

A sewage treatment system comprises a fixed bacteria bed and, upstream of the bacteria bed, a moving bed biofilm reactor. Preferably the moving bed biofilm reactor contains buoyant media having surfaces on which microbial growth can occur. The buoyant media may consist of small plastic cylinders having cross-shaped dividers on their inside and longitudinal fins on their outside. The buoyant media are caused to move by either aeration (in an aerobic reactor) or by a mechanical stirrer (in an anaerobic reactor).

Description

SEWAGE TREATMENT Field of the Invention This invention relates to the treatment of sewage, and to apparatus for use in such treatment.

Backaround of the Invention Sewage is typically treated by distributing it over fixed beds of either stones or plastics material (bacteria beds, or trickling filters). The process is used at many sewage works in the UK, but suffers a number of disadvantages. In particular, the construction cost is high, the process can have poor performance for nitrification, and high loading can cause blockage of surface layers, due to excessive biofilm growth.

EP-A-0776863 (published 04.06.97) discloses that the sludge age of nitrified activated sludge can be dramatically reduced by pre-treatment in a moving bed biofilm reactor (MBBR). In this combination, the MBBR apparently removes most of the biological oxidation demand (BOD) from the waste water input; this makes it possible for the activated sludge stage to carry out nitrification only. This occurs without the formation of a highly-loaded thick film, but rather with a thin, active film that has little or no anoxic/anaerobic material.

Summary of the Invention According to the present invention, a MBBR is provided, prior to a bacteria bed. By using this novel combination, the performance of the bed is enhanced. In particular, the MBBR can treat high loadings, and reduce the strength of effluent passing to the bed. Removal of BOD can prevent bed blockage, and allow nitrification to proceed effectively.

The invention provides effective, economic sewage treatment. It is of value in new systems and is also adaptable to the retrofitting of existing plants, to enhance existing processes.

Description of the Invention The apparatus and method of this invention are essentially conventional, in bacteria bed sewage treatment, except for the MBBR. In a moving bed biofilm reactor, the biomass grows on small carrier elements that move along with the water in the reactor. The movement is caused by aeration in an aerobic version of the reactor, and by a mechanical stirrer in an anoxic/anaerobic version. The reactor provides high specific biofilm surface, insignificant headloss through the system, and no need for backwashing or recycling the biomass.

Preferably, the MBBR comprises a tank which, in addition to an inlet and an outlet for the sewage under treatment, comprises an inlet for oxygen (or air containing oxygen) to be introduced under pressure. Within the reactor, a buoyant medium is present, providing surfaces on which microbial growth can occur; agitation of the elements controls biomass which allows good oxygen penetration. The medium preferably comprises discrete pieces of, say, plastics material, which preferably include an internal surface, such that solids are not removed by attrition between the pieces. An especially suitable material of this type is known as "Kaldnes" medium, and is described in detail in EP-A-0575314. Each carrier element of this medium is a plastic cylinder with cross-shaped dividers on its inside and longitudinal fins on the outside. The plastic cylinders are, for example, 9 mm in diameter and 7 mm long. They may be made from high density polyethylene, and have a specific weight between 0.92 and 0.96 kg/m3. As the carrier elements are lighter then water, the mixing is provided by air being blown into the reactor. In the case of anoxic or anaerobic reactors, a mechanical stirrer or propeller is used to keep the waste water and carrier elements moving.

The effective (i.e. inner) surface area of the elements is 500 m2/m3 in bulk. The total surface area for a Kaldnes element is much higher but, because of abrasion between the carrier elements when mixed, the bacterial growth is much weaker on the outer surfaces and is therefore not taken into account. The maximum percentage of carrier elements to an empty reactor volume is 70% which corresponds to a effective surface area of 350 m2/m3 of reactor. In the case where the reactor is filled to 50%, this gives an effective biofilm area of 250 m2/m3.

In a bacteria bed, carbonaceous removal occurs first, followed by nitrification. Nitrification therefore occurs at depth in the bed, e.g. 1.2-1.5 m down. Typically, beds are 1.8 m deep. In cold weather, processes occur more slowly, and the depth at which processes complete increases. By providing the MBBR before the bacteria bed, a reduction in BOD concentrations can therefore be effective in improved nitrification performance can be obtained.

The MBBR is uniquely appropriate for this use, because the solids produced are finely divided and not likely to cause blockage of the bacteria bed. Further, the process is continuously aerobic, encouraging nitrifiers on the bed.

In addition, the MBBR can take high loadings, creating a compact plant.

A problem typical to bacteria beds is blockage of the surface layers due to a thick biofilm. This thick biofilm occurs due to high concentrations of soluble BOD in the feed. The MBBR prior to the bed removes soluble BOD and prevents blockage.

Claims

1. A sewage treatment system comprising a bacteria bed and, upstream of the bacteria bed, a moving bed biofilm reactor.

2. A system according to claim 1, in which the reactor comprises a buoyant medium and a forced oxygen supply.

3. A system according to claim 2, in which the medium comprises discrete pieces of plastics material each having an internal surface.

4. A method for the treatment of sewage on a bacteria bed, which comprises the prior step of passing the sewage through a reactor as defined in any preceding claim.