WO2012087151A1 - Carrier element for purification of water - Google Patents

Abstract

It is described a carrier element for microorganisms attached in the form of a biofilm, used for biological treatment of water or wastewater, The carrier element angle (1) is 40 to 80 degrees off of the horizontal line or plane (2) that runs through the open area of the carrier. The height (4) of the carrier element is 7 - 20 mm. The width (5) of the carrier element is 15 - 50 mm. The carrier element has internal partition walls (3) providing a protected biofilm surface area of 400 - 950 m²/m³ of carrier elements, the carrier element has a void volume (pore volume) of at least 75 %, more preferred 80 %, most preferred 85 %; and the carrier element has a density of 0.90 to 1.45.

Description

CARRIER ELEMENT FOR PURIFICATION OF WATER.

The invention relates to a carrier element, upon which microorganisms attach in the form of a biofilm, and these microorganisms are used to biologically treat water or wastewater.

In biological treatment of water or wastewater the water is passed through a reactor or vessel where microorganisms are used to convert the impurities in the water to harmless end products. Biological processes can be aerobic, anoxic or anaerobic. In aerobic processes the microorganisms need molecular oxygen as electron acceptor and are supplied with air or pure oxygen. Aerobic biological treatment converts organic matter to carbon dioxide (C0₂) and water. In anoxic processes the microorganisms will use nitrate or sulphate as electron acceptors, when molecular oxygen is not present. For conventional biological nitrogen removal an aerobic process, where ammonium is oxidized to nitrate, is combined with an anoxic process, where nitrate is reduced to inert nitrogen gas. Anaerobic processes take place in an

environment free of oxygen and the organic material acts as both electron donor and electron acceptor. Anaerobic processes are mainly used for treating industrial effluents with very high concentrations of organic matter, or for digestion of sludge. Anaerobic processes convert organic material to a mixture of methane and C0₂ (biogas).

The microorganisms in a biological reactor can either be in a suspension in the reactor, or attached to surfaces as a biofilm. These two main categories of biological treatment are known as suspended growth processes and biofilm processes, respectively. Biofilm processes in general can handle higher loads and are less sensitive to variations and disturbances than suspended growth processes.

In suspended growth processes the microorganisms responsible for treatment are separated from the treated water in a downstream separation process and returned to the biological reactor, where they are maintained in liquid suspension by appropriate mixing methods. The most common suspended growth process is the activated sludge process. In biofilm

processes the microorganisms grow on a surface. Examples of biofilm processes used for wastewater treatment are trickling filters, Rotating Biological Contactors (RBCs), submerged biological filters, moving bed processes and fluidized bed processes. Submerged biological filters are biofilters with relatively open plastic media fixed in place, as well as biofilters with small diameter biofilm carriers (sand, Leca, polystyrene beads). Because the microorganisms in biofilm processes are attached to surfaces inside the biological reactor, the bioreactor function is independent of downstream separation processes.

Combinations of suspended growth processes and biofilm processes in the same reactor are known as IFAS (Integrated Fixed-film and Activated Sludge) processes. Activated sludge has been combined with either RBCs, high void biofilm media (trickling filter type media) or moving bed type biofilm media to create IFAS processes.

Traditional trickling filters were the first biofilm process used for wastewater treatment. Trickling filters originally used stones, but modern trickling filters are filled with plastic materials with larger specific surface areas for biofilm growth. Modern trickling filters are relatively high. Water is pumped to the top of the filter and evenly distributed over the entire surface area. Oxygen is supplied via natural ventilation. It is difficult to adjust water flow, pollution loads and natural oxygen supply in order to obtain optimum conditions. It is fairly common that the biofilm in the upper part of a trickling filter is oxygen limited. Thus, trickling filters normally have lower removal rates and require larger reactor volumes than other biofilm processes. To prevent clogging the plastic packing must be fairly open and the specific biofilm surface area (m² biofilm per m³ reactor volume) will be fairly low, thereby contributing to a large bioreactor volume. Even with an open packing material channelling and clogging of trickling filters are well known problems.

Rotating Biological Contactors (RBCs) were very popular in the 1970s and have plastic biofilm media attached to a horizontal, rotating axle. The biofilm media are partially submerged in the wastewater and continuously rotate, providing intermittent contact of the biofilm with the wastewater and atmospheric oxygen. Major technical problems have been experienced, often due to a lack of biofilm control leading to thick biofilms and too much weight, resulting in broken axels and/or failure of the plastic media. Hardly any new RBC plants have been built over the last 20 years.

Submerged biological filters with small diameter biofilm carriers have small voids that will rapidly be filled up and clogged from influent suspended solids and biomass growth, and they must routinely be taken off stream for backwashing and removal of sludge. They are known as Aerated Biological Filters (BAFs). Civil works reactor design is fairly complicated, in order to accommodate all the equipment needed for aeration and backwashing.

Submerged biological filters with high void biofilm media normally use the same biofilm media as in modern trickling filters. The media is stationary, submerged in the reactor, and air is supplied through diffusers at the bottom of the reactors. Submerged filters with large voids and stationary media are supposed to be operated with continuous feed of water and no backwash. However, experience has shown that a problem with this type of submerged biofilters has been clogging and channelling. Water and air follow the route of least resistance, creating dead zones in the media where biomass accumulates and cause anaerobic conditions. Another problem is limited or no access to the air diffusers. For maintenance or replacement of aerators the stationary biofilm media need to be removed. Predators (snails and worms) have also been a major problem with these systems.

In fluidized bed processes the biofilm grows on sand particles.

Wastewater is pumped in to the bottom of the reactor with sufficient speed to fluidize the sand. Fluidized beds have a very large biofilm surface area, measured at over 3300 m²/m³, and in aerobic processes it is difficult to supply enough oxygen. Normally the wastewater is recycled several times in order to get a sufficiently high flow rate to fluidize the sand, and oxygen is supplied by saturating the recycled water with air or pure oxygen. Pumping costs may be very high and in large full-scale plants it is very difficult to distribute the water in such a way that an even fluidization of the entire bed can be achieved. It is also a problem that biomass attached to the sand particles changes the specific gravity of the particles, so that sand with more biofiim fluidizes at significantly lower water flows than sand with less biofiim. Thus, operating a plant without loosing sand and biomass is a challenge.

In moving bed reactors the biofiim grows on carriers that move freely in the reactor. Biofiim carriers have either been polyurethane foam cubes or small plastic elements. Processes using foam cubes are known under the trade names Captor or Linpor. The disadvantages of foam cubes are that the effective biofiim surface area is small, because the growth on the outside of the cubes clog the pores and prevent substrate and oxygen from penetrating to the inner part of the cubes. Furthermore, sieves are required to prevent the foam cubes from leaving the reactors, and a system that constantly pumps foam cubes away from the sieves and back to the reactor inlet is required to prevent clogging of the sieves. As a result very few plants have been built with foam cube technology, with a total of 32 plants worldwide (including 21 plants in Germany) by the year 2000.

However, a lot of Moving Bed Biofiim Reactor (MBBR) plants that use small plastic carriers have been built in recent years. One vendor (Kriiger

Kaldnes) alone has sold more than 500 large plants in 45 countries. The plastic biofiim carriers move freely in the reactor volume and are kept within the reactor by a sieve arrangement at the outlet. Carriers typically have a density slightly lower than for water and bulk volumetric filling of carriers are typically up to 67 %. The process is continuous, with no need for backwashing and no sludge return. The MBBR process is very flexible with regard to reactor shape. Specific biofiim surface areas are higher than for trickling filters and RBCs, but significantly lower than for BAFs. However, on a volumetric basis MBBRs are know to be just as efficient as BAFs when considering the extra volumes needed for expansion of the BAF filter bed during backwashing and for wash water storage.

The Continuous Flow Intermittent Cleaning (CFIC^®) biofiim reactor process is a newly patented technology (Patent no 329665, PCT WO 2010/140898 A1). The CFIC^® reactor contains highly packed biofilm carriers to a degree (typically 90-99% bulk volumetric fill) that little movement of the carriers occurs in the reactor during normal operation. The CFIC^® process has continuous inflow to the bioreactor and intermittent cleaning, using influent wastewater, which removes excess biomass (sludge) from the biofilm carriers and provides active biofilm control. A carrier shape is needed that provides a large void volume (typically an 85% void volume in a 100% fill situation) for growth and accumulation of biomass. The void volume maximizes the run time between cleaning cycles. During cleaning cycles the water level in the reactor is elevated just enough to provide free movement of the carriers. The turbulence in the reactor may temporarily be increased to facilitate removal of biomass, and the biomass will be washed out of the reactor using influent wastewater.

A multitude of plastic biofilm carriers exist that may be used for trickling filters, submerged biological filters, MBBRs, CFIC^® and I FAS processes. The most commonly used types of plastic are high density polyethylene (HDPE) with a density of 0.94-0.97, low density polyethylene (LDPE) with a density of 0.91- 0.93, poly propylene (PP) with a density of 0.90-0.91 , PVC with a density of 1.35- 1.45, and ABS plastics with densities of 0.99 - 1.10. Biofilm carriers are available with protected surface areas from < 100 to 1200 m²/m³ bulk volume of earners.

For optimum performance the CFIC^® biofilm reactor process needs a biofilm carrier that has a high biofilm surface area, sufficient openings to provide good exchange of water (and air in aerobic reactors), a high void volume for storage of biomass, sufficiently large size to be retained in the reactor by outlet sieves, and a design that makes it easy to provide biomass control by removing the desired amount of biomass from the carriers during the cleaning cycle of the CFIC^® process. Prior art biofilm carriers used for BBR and IFAS processes have certain deficiencies that make them less desirable for use with the CFIC^® process. They either have insufficient biofilm surface area (PCT/SE95/00260), insufficient void volume for storage of biomass (PCT/SE03/00265), or they are small (PCT/NO91/00007) and will need small openings in the outlet grid or sieves. Thin carriers (PCT/SE03/00265) may escape through longitudinal openings in sieves and will require sieves with circular or square openings. Some carriers may also have a tendency to clump together and form clusters (PCT/NO91/00007; PCT/SE03/00265), making transfer of substrates (and oxygen) from the bulk liquid to the biofilm less efficient.

Figure 1 shows an example of a biofilm carrier element, produced according to the present invention. The carrier elements can be round, square or polygonal in shape. The innovative design would be to cut or mold the carrier elements with an angle (1 ) of 40 degrees to 80 degrees off of the horizontal line or plane (2) that runs through the open area of the carrier. In an MBBR or IFAS type process this design would create unbalanced surface areas that would be subject to uneven hydraulic and pneumatic forces that would cause the carrier element to rotate around an axis in the center from one side through to the other. This rotation would create and improve the flow of dissolved nutrients and gases through the cells of the carrier element giving more complete distribution of these dissolved nutrients over the biofilm surfaces.

During normal operation of a CFIC process the present invention prevents the carrier elements from sticking together end to end and thus impeding the flow of nutrients and gases through the elements, as well as preventing dead zones and channeling in the reactor due to areas where the carrier elements are being stacked together too tightly. The present invention ensures that the carrier elements, after each cleaning cycle, will be randomly packed in the reactor and provide a short distance from the bulk liquid to the biofilm on the carrier elements in all areas of the reactor, as well as providing lots of void volume, evenly distributed throughout the reactor, where biomass can accumulate. During the cleaning process, with an increased water level that enables free movement of the carrier elements, the design would create unbalanced surface areas that would be subject to uneven hydraulic and pneumatic forces that would cause the carrier element to rotate around an axis in the center from one side through to the other. This rotation would create and improve the flow of water and gases through the cells of the carrier element and efficiently dislodge excess sludge (biomass growth and particles settled inside the carrier element) from the carrier element.

Examples of carrier elements according to this invention are shown in Figure 1 and Figure 2. The carrier elements can be molded or extruded. The density can be from 0.90 to 1.45, depending on the application. The carriers can be used for aerobic biological treatment, anoxic biological treatment and anaerobic biological treatment.

Claims

C l a i m s

1.

Carrier element for microorganisms attached in the form of a biofilm, used for biological treatment of water or wastewater, characterized by: carrier element angle (1 ) of 40 degrees to 80 degrees off of the horizontal line or plane (2) that runs through the open area of the carrier, more preferred 45 to 75 degrees, most preferred 50 to 70 degrees; carrier element height (4) of 7 mm to 20 mm, more preferred 7 to 15 mm, most preferred 8 to 12 mm; carrier element width (5) of 15 mm to 50 mm, more preferred 15 to 40 mm, most preferred 20 to 35 mm; carrier element with internal partition walls (3) providing a protected biofilm surface area of 400 m²/m³ of carrier elements to 950 m²/m³ of carrier elements, more preferred 500 to 900 m²/m³ of carrier elements, most preferred 600 to 800 m²/m³ of carrier elements; carrier element with a void volume (pore volume) of at least 75 %, more preferred 80 %, most preferred 85 %; carrier element with a density of 0.90 to 1.45, more preferred 0.90 to 1.10, most preferred 0.94 to 1.10.

2.

Carrier element according to claim 1, characterized by the carrier element being kept in suspension and continuous movement in a moving bed biofilm reactor (MBBR) type process.

3.

Carrier element according to claim 1, characterized by the carrier element being kept in suspension and continuous movement in an integrated fixed-film and activated sludge (I FAS) type process.

4.

Carrier element according to claim 1 , c h a r a c t e r i z e d by the carrier element being used in the CFIC^® process.

5.

Carrier element according to claim ^characterized by the carrier element being used in aerobic reactors according to claims 2, 3 or 4.

6.

Carrier element according to claim 1, characterized by the carrier element being used in anoxic reactors according to claims 2, 3 or 4.

7.

Carrier element according to claim ^characterized by the carrier element being used in anaerobic reactors according to claims 2, 3 or 4.

8.

Carrier element according to claim 1, characterized by the carrier element being produced by plastic extrusion.

9.

Carrier element according to claim 1, characterized by the carrier element being produced by plastic molding.