GB2613232A

GB2613232A - Wastewater treatment system

Info

Publication number: GB2613232A
Application number: GB2214113.9A
Authority: GB
Inventors: Baird Andrew
Original assignee: Wcs Env Engineering Ltd
Current assignee: Wcs Env Engineering Ltd
Priority date: 2021-09-27
Filing date: 2022-09-27
Publication date: 2023-05-31
Also published as: GB202214113D0; GB202113803D0

Abstract

A wastewater treatment system 10 includes an aeration chamber 12 and a diffuser system 30 for supplying gas, such as air or oxygen gas, into the chamber. The diffuser system both aerates and sets up a rotational flow of the wastewater being treated. The rotational flow is directed along at least one longitudinally-extending lighting unit 32a, 32b, which illuminate wastewater in their vicinity. The diffuser system may include a diffuser head 40 with a plurality of apertures positioned within a draft tube 24. Suitably, each lighting unit emits light in a spectral range that is used in photosynthesis and may include at least two LED strip lights. By recirculating wastewater, photosynthesising species carried within the wastewater are provided with repeated, intermittent exposure to light. This encourages algal growth, fed by nutrients such as phosphorus- and nitrogen-containing compounds. Hence, the system provides an environment that simultaneously encourages two biological processes within the single chamber: bacterial digestion of organic pollutants and the extraction of biological nutrients by algae photosynthesis. A diffuser assembly is also claimed, comprising a hollow pipe 38 connecting the diffuser head and a gas inlet, wherein a lighting unit extends in a longitudinal direction adjacent to the hollow pipe.

Description

WASTEWATER TREATMENT SYSTEM

This invention relates to the field of wastewater treatment and, in particular, to aerobic waste treatment systems in which development of aerobic microorganisms is promoted in order to reduce the polluting load.

Packaged wastewater treatment systems, such as that described in WO 2014/207433, are common in the UK, largely replacing septic tanks as the preferred option for the disposal of wastewater from properties not connected to a mains sewage system. In comparison with the septic tank, a packaged wastewater treatment system is better able to meet the more stringent demands being placed by regulatory authorities on the quality of treated water being discharged into the environment, either to land or in water. These systems generally use bacteria to digest contaminants and organically break down impurities into carbon dioxide and water.

To date, the focus of measures of the quality of effluent water discharged from domestic wastewater treatment systems has been on the reduction of contaminants such as organic pollutants, suspended solids and ammonia. An indication of organic pollutants is provided by the Biological Oxygen Demand, 5-day test (BOD5), which measures the decrease in the concentration of oxygen as it is consumed by bacteria that decompose the organic matter. Standards demand that in order for the effluent to be discharged to the environment, impurity levels must, typically, be below 20mg/I BOD5, 30mg/I suspended solids and 20mg/I ammonia. Biological nutrients however, such as nitrogen-and phosphorus-containing compounds, can also be harmful to the environment and their presence in in domestic wastewater is being observed in increasing concentrations.

Nutrient enrichment (eutrophication) of rivers and lakes leads to an overabundant growth of water plants, which can present problems for ecosystems: depleted oxygen levels leading to the death of aquatic animals, murky water and depletion of desirable flora and fauna.

Limits to the level of phosphorus present in the effluent from wastewater treatment plants run by water utility companies are gradually being imposed in England. Larger plants presently meet the requirements using a combination of biological and chemical treatments. The smaller sites are looking to comply, with proposed treatments generally being chemically-based: dosing with either aluminium-or iron-based salts in order to precipitate phosphorus. There is however no suitable technology available to achieve phosphate removal down to acceptable levels in domestic or commercial packaged wastewater treatment systems. Chemical dosing has been attempted but the possibility of metal salts being discharged potentially poses a greater risk to the natural environment than that of the contaminants the chemicals seek to remove. In lightly-populated areas the impact may be acceptable but in areas with a high population density, the cumulative effects of metal salts can cause severe damage to the marine environment.

There is therefore a perceived need for an alternative method of removing biological nutrients from wastewater, which is suitable for implementation in packaged treatment systems for the domestic and commercial market.

The present invention accordingly provides a wastewater treatment system that includes an aeration chamber and a diffuser system for supplying gas into the chamber, the diffuser system being configured to supply gas in a manner that sets up a rotational flow of wastewater contained within the chamber. The invention is characterised in that the system also includes at least one longitudinally-extending lighting unit that is oriented substantially in line with a section of the rotational flow of the wastewater that the diffuser system is configured to set up.

This combination of rotational flow and lighting establishes conditions in the aeration chamber that are conducive to photosynthesis. The rotational flow that is set up causes wastewater within the chamber to pass over the lighting units multiple times per minute. Consider a volumetric unit of liquid within the chamber. At one point during its rotational cycle the flow will be such that this unit is channelled along the length of the lighting unit. The recirculation provided by the flow pattern that is set up means that the same volumetric unit is, at intervals, brought back into the vicinity of the lighting unit. As it flows along the length of the lighting unit, any photosynthesising microorganisms contained within that volume of wastewater pass sufficiently close to the light source that they are also exposed to light. This intermittent exposure at the frequency established by the rotational flow and the duration for which traversal of the longitudinally-extending light lasts is sufficient for photosynthesis. Algae and any other photosynthesising species that are naturally present in wastewater are therefore encouraged to grow. Such growth is fed by nutrients such as phosphorus-and nitrogen-containing compounds, which are accordingly removed from the wastewater.

Although nutrient removal by algal photosynthesis is known in the prior art, systems that implement this process generally require a clear effluent in order to permit sufficient transmission of light to facilitate growth of the algae. Wastewater is highly turbid and accordingly there has not, to date, been a mechanism found by which photosynthesising species within this medium may be provided with sufficient light for photosynthesis. The present invention solves this problem by establishing a rotational flow that recirculates wastewater such that it passes along the length of an elongate light source multiple times per minute. With each pass, photosynthesising microorganisms are brought within the limited penetration distance of light from the source and so accumulate an exposure that is sufficient to enable photosynthesis. In exploiting this mechanism, this invention therefore provides a system that facilitates biological removal of harmful nutrients in a packaged treatment system, such as may be suitable for use in both domestic and commercial markets.

In such a packaged wastewater treatment system, the driving gas that sets up the rotational flow of wastewater is air. This system therefore provides an environment that simultaneously encourages two biological processes that operate within the single chamber to process wastewater. The first process is bacterial digestion of organic pollutants, which makes use of the oxygen that is supplied by the driving gas. The second is the above-described extraction of biological nutrients by algae photosynthesis. Moreover, these two biological processes are to some extent symbiotic.

Carbon dioxide is a by-product of bacterial activity, and this is used in photosynthesis. Oxygen, which is essential for bacterial growth, is generated by photosynthesis.

In order to promote photosynthesis, each lighting unit ideally emits light that is concentrated in a spectral range that is used in photosynthesis.

Preferably, each lighting unit comprises at least two LED strip lights. This provides a low-power implementation of a longitudinally-extending lighting unit that can be fabricated to emit in the spectral range required. That is, a traditional aerobic bacterial treatment plant can be adapted to provide the further benefit of nutrient removal with little additional power consumption.

In one preferred arrangement, the lighting units are oriented such that their longitudinal direction is substantially vertical. They may be laterally separated and distributed within the aeration chamber. For example, two units, one either side and forward of a central axis; or four units, one in each chamber quadrant. Although the light from these sources does not penetrate far into the turbid effluent, and so there would seem little reason to separate them, closely spaced lighting units may each act as a source of disruption to flow patterns around neighbouring units. In wastewater treatment systems of the type that include an air lift pump to aerate and mix the effluent, the lighting unit may be positioned within a tubular part of the pump, close to an air outlet. In particular, for systems in which the diffuser system includes a diffuser head with a plurality of apertures that is positioned within a draft tube, another preferred arrangement is for the lighting unit to be positioned within the draft tube, extending towards the diffuser head. This arrangement is doubly advantageous. First, adapting the diffuser system means that this invention can be implemented in existing systems simply by replacing a traditional diffuser system with one with an integrated lighting unit. That is, no modification of tanks that may already be installed underground would be required. Secondly, the flow is better directed within a channel created by the draft tube wall. This ensures that the longitudinal direction of the lighting unit is aligned more exactly with flow direction, which in turn leads to an arrangement that maximises residency time of a volumetric unit of wastewater in the vicinity of the lighting unit. In one preferred embodiment in which the diffuser system includes a hollow pipe that connects a gas inlet to the diffuser head, the lighting unit is preferably attached to an external surface of the hollow pipe.

The diffuser system and draft tube may be substantially vertically oriented, with the draft tube extending through a lower aperture at the base of the aeration chamber. Moreover, the diffuser head may be located at a position within the draft tube such that it is below the lower aperture of the aeration chamber. This arrangement is advantageous in ensuring that the draft tube acts as an air lift to maintain well-oxygenated waste that is mixed to create a homogeneous effluent that is ideal for bacterial growth. At the same time, it helps maintain the flow pattern required for photosynthesising organisms within the effluent to be exposed to sufficient light.

In a second aspect, the present invention provides a diffuser assembly for fitting to a wastewater treatment system. The diffuser assembly includes a diffuser head with a plurality of apertures; a hollow pipe connected at one 30 end to the diffuser head and configured at the other end to connect to a gas inlet; and a lighting unit that extends in a longitudinal direction adjacent to the hollow pipe, the lighting unit containing lights that emit in a spectral range that is used in photosynthesis. In this aspect, the invention provides an adapted diffuser assembly that may be fitted to previously-installed packaged treatment systems and so allow them to provide the benefits of this invention. That is, it enables existing treatment systems based on bacterial digestion of organic waste to additionally remove harmful phosphorus-and nitrogen-based salts from wastewater. In use, the diffuser assembly is preferably of the type that is fitted within a tubular part of an air lift system, such as a draft tube, that extends through a lower aperture at the base of an aeration chamber of a wastewater treatment system. Channelling of wastewater flow pattern within the draft tube is found to assist with alignment between the flow direction and longitudinal extent of the lighting unit, maximising residency time of each wastewater unit within an illumination zone that penetrates only a short distance from the lighting unit into the wastewater.

In another aspect, the present invention provides a method of treating wastewater in a bioreactor chamber, the method including the steps of: a) Establishing a rotational flow of the wastewater in the chamber such that a section of the rotational flow is directed along at least one longitudinally-extending lighting unit located in the chamber; and b) Emitting light from the at least one lighting unit so as to illuminate wastewater in the vicinity of the lighting unit.

This method accordingly provides a mechanism by which photosynthesising algae, and similar organisms, that are naturally present in wastewater, are provided with sufficient light to encourage photosynthesis, notwithstanding the turbidity of wastewater. As a consequence of photosynthesis, biological nutrients are removed from the wastewater before the treated water is released into the environment.

The step of establishing the rotational flow includes the step of pumping oxygen gas or air through a plurality of apertures in a diffuser head so as to cause rapid airflow upwards within a draft tube (or air lift configuration) in which the diffuser head is located. Such oxygen flow aerates the wastewater, encouraging the action of aerobic bacteria whose digestive action is responsible for the breakdown of organic waste contained within the wastewater to harmless products: carbon dioxide and water.

Ideally, the step of emitting light from the at least one lighting unit includes emitting light in a spectral range that is used in photosynthesis.

In one preferred embodiment, the step of illuminating wastewater includes illuminating wastewater within the draft tube, this being achieved by locating the longitudinally-extending lighting unit in the draft tube above the diffuser head.

Preferably, airflow within the draft tube or air lift configuration is in the range of 1.5 -2.5 ms-1 and, more preferably, around 2ms-1. The step of establishing the rotational flow of the wastewater in the chamber may include establishing the rotational flow within the range of 20 to 40 litres of wastewater per second. Similarly, the rotational flow may turn over chamber capacity between 3 and 50 times per minute.

After a period of operation, which may be up to 3 years, a maintenance procedure should be carried on the bioreactor chamber. This preferably includes the step of removing algae and bacteria from the chamber. In larger treatment systems a separate final settlement tank may be provided as part of the packaged plant. In this variation, treated water is stored in the settlement tank while algae and bacteria settle out, for subsequent removal from the system.

The invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is an internal view (vertical section) of a first embodiment of a wastewater treatment system in accordance with the present invention; Figure 2 is a top view of the wastewater treatment system shown in Figure 1, Figure 3 is an illustration of the flow pattern set up within the wastewater treatment system of Figures 1 and 2, and Figure 4 is an internal view (vertical section) of a second embodiment of a wastewater treatment system in accordance with the present invention.

With reference to Figure 1, a wastewater treatment system 10 in accordance with this embodiment of the invention is based on an activated sludge process. The system includes an aeration chamber 12 suspended within an outer tank 14. The outer tank 14 is closed with a cover 16 and removable lid 18. The aeration chamber 12 has a lower aperture 20 at its base and an upper aperture 22 at its top to allow fluid flow between the aeration chamber 12 and outer tank 14. A draft tube 24 has a plurality of fins 26 arranged circumferentially around a central portion of the tube. The draft tube 24 is sized such that it fits within the aeration chamber 12 with the fins 26 resting against a lower inside surface 28 of the chamber 12.

This allows a lower portion of the draft tube 24 to protrude outside the aeration chamber 12, with the upper portion extending into a lower section of the chamber 12. A diffuser assembly 30 is centrally positioned within the system and extends from the cover 16 through the aeration chamber 12 and a substantial distance into the draft tube 24, such that it extends below the fins 26. A pair 32a, 32b of tubular lighting units are oriented vertically within the aeration chamber 12 and outside the draft tube 24. An inlet pipe 34 for conveying wastewater into the system, extends through the outer tank 14 and into an upper part of the aeration chamber 12. An outlet pipe 36 (dip pipe) facilitates removal of treated water from an upper part of the outer tank 14 and out of the system 10.

An air inlet is located in the cover 16 for connection with an external air pump (not shown). The diffuser assembly 30 includes a narrow hollow pipe 38 that connects the air inlet to a diffuser head 40 that is positioned within the draft tube 24. The diffuser head 40 includes a plurality of apertures and is therefore able to deliver air inside the draft tube 24 at a flow rate determined by the pump.

Each lighting unit 32a, 32b is a sealed tube containing two 750 mm strip lights. The 750 mm in this size (845 litres) and design of aeration chamber 12 is sufficient for the lights to extend from the level of the outlet pipe 36 to a shod way below the upper opening from the draft tube 24. That is, to ensure illumination of a significant depth into the wastewater, whilst avoiding unnecessary surface illumination. Clearly, with larger volume chambers 12 it will be possible to make use of longer strip lights. The lights themselves are hydroponic or grow lights. That is, they are specifically designed to give off light in a spectral range that is most useful for photosynthesis. In this embodiment, LED lights are used, with a light output in the range 2400 -4800 lumens. LED lights tend to be lower power (around 50 W for this light output) than alternative light sources and so are preferred for this reason. It is the light output though (spectrum and intensity) that is important for this invention and alternative sources can be used. Cabling 42 extends through sealed holes in the cover 16 to the lighting units 32a, 32b in order to connect them to an external power source, which may be mains electricity. As shown in Figure 2, the two lighting units 32a, 32b are offset from a central axis of the aeration chamber 12.

As noted above, the wastewater treatment system of this embodiment is -10 --based on an activated sludge process. This requires seeding the chamber prior to a treatment cycle with bacteria-containing sludge that is recovered during a previous cleaning and maintenance cycle.

With reference to Figure 3, the operation of the wastewater treatment system 10 will now be described. The diffuser assembly 30 is connected to a supply of air 44 and the lighting units 32a, 32b are switched on. Wastewater On the form of domestic sewage or raw sewage) is fed into the aeration chamber 12 via the inlet pipe 34. Solid waste 48 and wastewater at the basal region of the outer tank 14 is drawn into the draft tube 24 where it is mixed and lifted into the aeration chamber 12 by the action of air as it is forced out of the diffuser assembly 30 at a speed of around 2 ms-1. This establishes a flow pattern within the wastewater, as indicated by arrows 46. The airflow from the diffuser assembly 30 lifts the wastewater rapidly upwards through the draft tube 24, propelling it into the aeration chamber 12. The walls of the chamber guide the wastewater as it then circulates about the aeration chamber 12 flowing upwards, radially outwards and then downwards before repeating its upward flow at the central region. The flow rate of the wastewater within the chamber 12 is in the range of 20 -40 litres per second. The tubular lighting units 32a, 32b are oriented vertically within the chamber 12 so as to be substantially parallel with the flow direction as the wastewater passes their location. The circulation pattern established within the chamber therefore causes the wastewater to flow repeatedly along the length of the lighting units 32a, 32b. With a flow rate of 20 -40 litres per second, this repetition is between 10 and 50 times per minute. Water that is substantially free of solid waste is able to escape from the aeration chamber 12 to the clarifying chamber, formed by the region between the chamber and the outer tank 14. As it clarifies, treated water floats upwards within the outer tank 14 for removal from the treatment system via the dip pipe outlet 36. Flocculated micro-organisms (algae and bacteria) separate out of the treated water, are retained in the bioreactor and removed when the system is de-sludged as -11 -part of a standard maintenance regime.

Conditions in the aeration chamber are such that two biological processes are encouraged: bacterial digestion and photosynthesis. Bacteria are present in the chamber from the preparatory seeding. Wastewater contains naturally-occurring photosynthesising species, such as algae, and so these enter the chamber 12 with the wastewater via the inlet 34.

The action of the diffuser assembly 30 promotes oxygenation of the water in the sewage and so encourages bacterial activity to treat the waste trapped in the aeration chamber 12. As is well known, the natural bacterial digestion process by which organic waste is broken down to carbon dioxide and water is accelerated by conditions that are established within the chamber 12.

The flow pattern within the aeration chamber means that algae and similar organic matter within the wastewater are repeatedly exposed to hydroponic light from the lighting units 32a, 32b. This facilitates photosynthesis and the algae are able to grow by extracting nutrients (phosphorus and nitrogen) from the wastewater.

In the prior art, nutrient removal by algal photosynthesis is known. In these known applications though, clear effluent is required in order that sufficient light transmits through the medium to stimulate algal growth. Nutrient removal is therefore generally carried out as a separate stage after clarification in large-scale wastewater processing, for example in a municipal plant. Moreover, most systems that are currently operating rely on sunlight as a light source for photosynthesis, although in countries with insufficient sunlight such as the UK, artificial light sources have been used to supplement or replace this. Regardless of light source, it has not previously been possible to implement this process in a bioreactor, enabling small-scale operation in a packaged wastewater treatment system -12 -for the domestic or commercial market.

Wastewater is, by its very nature, highly turbid. This makes it extremely difficult to ensure any sort of light penetration into the medium, still less to a level required to facilitate photosynthesis and encourage algal growth. Any light source 32a, 32b placed in such an environment will create only a small illumination zone in its vicinity. With this present invention however, the rotational flow established by the pressurised aeration system means that the wastewater is constantly being recirculated. As a result, volumetric units of the wastewater will each periodically be brought into the illumination zones and so will intermittently be exposed to light.

Photosynthesising species carried within the wastewater are therefore also intermittently exposed to light. It is found that such exposure is sufficient for photosynthesis and therefore algal growth.

A further advantage of this invention is that the high flow velocity of the wastewater will tend to displace any solids that adhere to the light units, therefore maintaining illumination intensity throughout the process.

Flow rates and recirculation parameters in the aeration chamber 12 will be dependent on a number of factors including size of the packaged treatment system; air pressure; size and aperture arrangement of the diffuser. As an example, in a treatment system of the type described in WO 2014/207433 suitable for a 5-person population, the aeration chamber 12 has a volume of 845 litres. The upward flow rate generation by the diffuser aeration is 20 -40 litres per second, meaning that the chamber capacity is turned over approximately 3 times per minute. In other systems, the turnover rate is expected to be between 10 and 50 times per minute.

Experiment has shown that apparatus in accordance with this invention can consistently achieve phosphate removal rates of over 90%. Phosphorus concentrations in the treated water can be less than 1 mgl-1, although this -13 --does depend on the incoming concentration. Chlorophyll content in the aeration chamber 12 was found to be 5 -10 times that normally found in a standard domestic sewage plant. This evidences the growth of photosynthesising species, which are believed to be largely responsible for the removal of phosphorus and other nutrients. It is though possible that they may be supported in this by non-photosynthesising organic species that can also take up phosphates.

Similar rotational flows are established in other known wastewater treatment systems and it is envisaged that these can also be fitted with light units in order to encourage algal growth in the bioreactor. These systems may be domestic packaged wastewater systems, or commercial systems that treat water on a larger scale. It is the combination of wastewater flow with internal illumination that is important for this invention and size of plant is not seen per se as a limiting factor, although other considerations such as ease of establishing flow patterns and providing sufficient illumination sources in larger tanks will of course affect its implementation.

Suitable lighting units can be retrofitted to existing wastewater treatment plants in which aeration in order to encourage bacterial action is supported by a fast rotational flow. This, combined with the low additional running costs (50 W LED bulb in the embodiment described), makes this invention economic to implement. It is certainly less costly, both in terms of economy and health and safety risk, in comparison with chemical mechanisms to remove phosphates.

Figure 4 shows an alternative embodiment of this invention with components that are common to the first embodiment being similarly referenced. This embodiment differs from that shown in Figures 1 to 3 in that it has only a single lighting unit 50 and that extends centrally through the aeration chamber 12 and into the draft tube 24. This lighting unit 50 comprises a sealed tube containing six strips of 800 mm LED lights. As the -14 -light from the lighting unit 50 does not penetrate far into the wastewater, the proximity of the lighting unit 50 to the walls of the draft tube 24 is immaterial. Advantageously, wastewater in the draft tube 24 has higher flow velocity that is channelled in a more vertical direction by the tube wall, in comparison with that outside of the tube 24 and in the aeration chamber 12. This appears to be a more effective arrangement, perhaps because the higher flow velocity serves to keep the lights cleaner; perhaps because the tube wall ensures longer residency times in the illumination zone. Moreover, incorporation of a lighting unit 50 in the diffuser permits a less complicated retrofit, requiring only replacement of a standard air diffuser pipe 38 with a light-enhanced diffuser tube.

The arrangement and structure of lighting units is not constrained to be that shown in these embodiments. For example, four units may be distributed in a symmetric arrangement around the aeration chamber or the two shown in Figure 1 may be diametrically opposed. A greater number of lights would be advantageous in larger bioreactors. They should though be lights that emit in spectral ranges required for photosynthesis. Ideally, they should also be longitudinally extending in order to facilitate their orientation in line with a flow direction that is established within the aeration chamber, which in turn maximises flow path length within the illumination zone.

Although the embodiments described above focus on treatment systems that implement an activated sludge process, this invention is not limited to such systems. As will be apparent to one skilled in the art, it is the combination of lighting plus recirculating flow that is required to encourage photosynthesis and so this system can be implemented in other aerobic treatment systems that have rotational flow characteristics.

Claims

-15 -CLAIMS1. A wastewater treatment system that includes: a) An aeration chamber (12); and b) A diffuser system (30) for supplying gas into the chamber (12), the diffuser system (30) being configured to supply gas in a manner that sets up a rotational flow of wastewater contained within the chamber (12); characterised in that the system also includes at least one longitudinally-extending lighting unit (32a, 32b, 50) that is oriented substantially in line with a section of the rotational flow of the wastewater that the diffuser system (30) is configured to set up.
2. A wastewater treatment system according to claim 1 wherein each lighting unit (324 32b) emits light in a spectral range that is used in photosynthesis.
3. A wastewater treatment system according to claim 2 wherein each lighting unit (32a, 32b) includes at least two LED strip lights.
4 A wastewater treatment system according to any preceding claim that includes two laterally separated lighting units (32a, 32b) that are oriented such that their longitudinal direction is substantially vertical.
5. A wastewater treatment system according to any preceding claim in which the diffuser system (30) includes a diffuser head (40) with a plurality of apertures that is positioned within a draft tube (24).
6 A wastewater treatment system according to claim 5 wherein the lighting unit (50) is positioned within the draft tube (24), extending towards the diffuser head (40).
-16 - 7 A wastewater treatment system according to claim 5 or 6 wherein the diffuser system (30) and draft tube (24) are substantially vertically oriented, the draft tube (24) extending through a lower aperture (20) at the base of the aeration chamber (12).
8. A wastewater treatment system according to claim 7 wherein the diffuser head (40) is located at a position within the draft tube (24) such that it is below the lower aperture (20) of the aeration chamber.
9 A wastewater treatment system according to claim 6 wherein the diffuser system (30) includes a hollow pipe (38) that connects a gas inlet to the diffuser head (40) and the lighting unit (50) is fixed to an external surface of the hollow pipe (38).
10.A diffuser assembly (30) for fitting in a wastewater treatment system, the diffuser assembly (30) including a diffuser head (40) with a plurality of apertures; a hollow pipe connected at one end to the diffuser head (40) and configured at the other end to connect to a gas inlet; and a lighting unit (50) that extends in a longitudinal direction adjacent to the hollow pipe (38), the lighting unit (50) containing lights that emit in a spectral range that is used in photosynthesis.
11.A diffuser assembly (30) according to claim 10 wherein the diffuser assembly (30) is fitted within a draft tube (24) that extends through a lower aperture (20) at the base of an aeration chamber (12) of a wastewater treatment system.
12.A method of treating wastewater in a bioreactor chamber, the method including the steps of: a) Establishing a rotational flow of the wastewater in the chamber such that a section of the rotational flow is directed along at least one -17 -longitudinally-extending lighting unit (32a, 32b, 50) located in the chamber; and b) Emitting light from the at least one lighting unit (32a, 32b, 50) so as to illuminate wastewater in the vicinity of the lighting unit
13.A method according to claim 12 wherein the step of establishing the rotational flow includes the step of pumping oxygen gas or air through a plurality of apertures in a diffuser head (40) so as to cause rapid airflow upwards within a draft tube (24) in which the diffuser head (40) is located.
14.A method according to claim 13 wherein the longitudinally-extending lighting unit (50) is located in the draft tube (24) above the diffuser head (40), the step of illuminating wastewater therefore being illuminating wastewater within the draft tube (24).
15.A method according to claim 13 or 14 wherein the airflow within the draft tube (24) is in the range of 1.5 -2.5 ms-1.
16.A method according to claim 15 wherein the airflow within the draft tube (24) is around 2 ms-1.
17.A method according to any one of claims 12 to 16 wherein the step of emitting light from the at least one lighting unit (32a, 32h) includes emitting light in a spectral range that is used in photosynthesis.
18.A method according to any one of claims 12 to 17 wherein the step of establishing the rotational flow of the wastewater in the chamber includes establishing the rotational flow within the range of 20 to 40 litres of wastewater per second.
19.A method according to any one of claims 12 to 18 wherein the step of -18 -establishing the rotational flow of the wastewater in the chamber includes establishing the rotational flow to turn over chamber capacity between 3 and 50 times per minute.
20.A method according to any one of claims 12 to 19 wherein the method is followed by a maintenance procedure that includes the step of removing algae and bacteria from the bioreactor chamber.