The sewerage of many old buildings is bad. Thus, the waste water is, after sedimentation in a septic tank, often discharged directly into a stream, a lake, or a field drain.

Establishing a sewerage network will be costly, and the long transport to a large sewage treatment plant will result in generation of hydrogen sulphide.

However, ever increasing demands are today made on sewage treatment which is normally completed in large central plants in the surrounding sewage area. First, the waste water settles in e. g. a septic tank before it is led to further treatment by active sludge method in large treatment plants.

The object of the invention is to provide a method of the kind mentioned in the opening paragraph, which can pass off more effectively than known so far when the invention is employed in open country.

A second object of the invention is to minimise the need for final clarification of the waste water and to reduce the total sludge production.

A third object of the invention is to minimise the space requirements of the treatment process and in a simple manner, to put it to use in connection with a septic tank in a household.

Thereby, an effective utilisation of the carrying elements is obtained which, within the process, always will be taking an active part in the biological process.

The novel and unique features according to the invention, whereby this is achieved, is the fact that the carrying elements only are filled into the riser pipe and are kept enclosed in this during the process.

The carrying elements can be in the shape of thin discs with a very large surface of between 80.000 and 150,000 m2/m3 as a hold to the biofilm whereby the biological treatment process can proceed within a confined area.

In an especially simple and advantageous embodiment of the method according to the invention, the waste water can be made to flow through the riser pipe by, during one period, sending an airflow in under the riser pipe and during a second period, sending the waste water in a flow through the riser pipe by means of a propellant, such as a pump or a propeller.

The waste water is hereby put into contact with the biofilm and gives its varying content of aerobic heterotrophic bacteria which digest organic material, autotrophic bacteria such as nitrosomonas, and nitrobacter, which digest nitrogen compounds to nitrate, and facultative bacteria which digest nitrate to free nitrogen good digestion conditions.

Additional waste water can be supplied to the container from a preliminary settling tank, such as a septic tank, at the same time as the treated water is led to a receiver. The method can therefore easily be employed the moment there is a need for treating a quantity of waste water.

The invention also relates to a plant for biological treatment of waste water and comprising a container with a quantity of waste water containing a large number of separate carrying elements, a riser pipe placed vertically in the container, and means for making the waste water flow through the riser pipe so that the waste water in the container is set in circulation.

In an especially expedient embodiment of the plant, the carrying elements can only be placed in the riser pipe where they advantageously are kept enclosed by means of a net at each end of the pipe, and thereby obtain an optimum efficiency.

When the carrying elements are kept enclosed in this way, the riser pipe with content appears as a compact unit which simply can be placed in or removed from a container with waste water.

The plant can therefore advantageously be placed alone or together with several identical units in already existing containers where there is a need for waste water treatment.

The waste water can be made to flow through the riser pipe by means of an air pump for, via an air pipe, blowing air in at the bottom opening of the riser pipe.

By airing the waste water in this way during prefixed periods, aerobic growth conditions can, in a simple manner, be maintained in the waste water while anaerobic conditions can, at least partly, be maintained when the airing is stopped. The air flow also contributes to keeping the carrying material with the biofilm fluidised.

By making the waste water flow through the riser pipe by means of e. g. a water pump or a propeller, a recirculation is obtained that constantly will put additional waste water into contact with the biofilm.

When the carrying material is kept in motion in this way, the separate carrying elements will constantly grate against each other. The biofilm can hereby be kept thin and thereby biologically active.

The air pump and the water pump can be controlled in a simple manner by means of an electric control system which has a timer so that the period of air supply and aerobic conditions, and the period with no air supply and anaerobic conditions both can be fixed within time periods that can result in an optimum biological digestion of the quantity and the type of the present waste water and hereby minimise the sludge quantity.

The invention will be explained below, describing additional advantageous characteristics and technical effects according to the invention with reference to the drawing, in which Fig. 1 is a vertical sectional view of a plant according to the invention, Fig. 2 is a horizontal sectional plan view of the plant, and Fig. 3 is a horizontal sectional plan view from another place on the plant.

In the following, a plant according to the invention is described on the exemplary assumption that the waste water is from an ordinary household, and that it first has passed a septic tank, in which large parts have settled.

Fig. 1 is a schematic view of a plant 1 which is mainly made up of a container 2 with an intake pipe for waste water 3, an outlet pipe for water 4, a bio zone 20 enclosed by a riser pipe 5 with a top and a bottom net 6 and 7 respectively of e. g. stainless steel. The riser pipe which contains a large number

of separate carrying elements 8 can, in this example, be a glass fibre case.

Air is supplied to the riser pipe 5 during one period by means of an air pump 12, e. g. a diaphragm blower, via an air pipe 13 ending at the bottom net 7 and said pump is spreading the air through a diaphragm 14, e. g. a diaphragm diffuser.

The water is kept in circulation during a second period by means of a pump 9, e. g. a submersible pump, which, via a water pipe 10 ending in a number of valves 11, pumps water in under the bottom net 7.

Airing and water flow is controlled by means of a timer 15 which is connected to a power supply (not shown) with a wire 18. The timer is connected to the water pump, e. g. a submersible pump, with a wire 16 and to the air pump with a wire 17.

Settled waste water is led through the intake pipe 3 to the container 2 which in the shown case is a concrete pipe, and is distributed in the riser pipe with the carrying elements 8. The carrying elements are covered by a biofilm, in which aerobic growth conditions are created by, during the first period, pumping air and thereby oxygen through the bio zone in the riser pipe 5.

By stopping the air supply, anaerobic conditions are created during a second period. The waste water is now flowing through the nozzles 11 and successively through the riser pipe 5 and is hereby put into good contact with the biofilm.

Both the airflow and the water circulation cause the carrying elements to be kept uniformly distributed in the riser pipe.

Fig. 2 is a schematically horizontal sectional view through the plant on a level with the water pump 9 where this is placed above the top net 6.

Fig. 3 is a schematically horizontal sectional view in which the water pump 12 and the timer 15 jointly are placed above the riser pipe 5 through the box 19.

As the air-and water-supply periods are electrically controlled by means of the timer 15, conditions are created for an effective biological digestion, and treated water can exit the container via the outlet pipe 4 which e. g. is placed on a level lower than the intake pipe 3.

The constant biological catabolism of supplied organic material reduces the strain considerably and reduces the need for further biological treatment in large wastewater treatment plants.

It is furthermore possible to employ the plant in connection with reconditioning or building existing plants together, such as trickling filters in which nitrogen, after all, is not sufficiently eliminated.