DESCRIPTION

"BIOLOGICAL PROCESS FOR WASTEWATER TREATMENT'

Field of the invention

The current invention is a biological process to treat wastewater that uses zeolites and can simultaneously carry out secondary and tertiary treatment. It is essentially constituted by one or more stages combined in series, each one of them having a chamber for aeration of the effluent with suspended biomass followed by a chamber with immobilized zeolites and also a module for biomass retention that separates the suspended biomass and the clarified effluent.

Background of the invention

As is well known, the treatment of effluents requires three stages of treatment: primary, secondary and tertiary treatment. Primary treatment reduces the suspended solids and biochemical oxygen demand (BOD) load in downstream treatment processes. Secondary treatment removes the dissolved and suspended organic matter and is normally processed through contact between microorganisms and the organic content of the effluent. Tertiary treatment has as its fundamental objective the removal nutrients such as

nitrogen and phosphorous .

Wastewater containing pollutants is usually- treated using biological processes. The process is characterized by the wastewater passing through some type of reactor, tank or basin where microorganisms are utilized for converting impurities existing in the water into harmless products such as carbon dioxide and water. The suspended growth method, know as the activated sludge process, is one of the most widely used biological processes . Most municipal wastewater treatment plants use this technology in their secondary stage for removing organic pollutants from the wastewater. The conventional activated sludge system comprises an aerobic tank (operating under aerobic conditions) and a clarifier (know as a secondary clarifier) . The wastewater and the activated sludge recirculated from the clarifier flow into the aeration tank. Air or oxygen is supplied to the aeration tank through an aeration system. In the aeration tank, the pollutants are either degraded or adsorbed by the activated sludge. The aeration tank mixed liquor then enters the clarifier for the solid-liquid separation phase. The clarified liquid is discharged through the clarifier outlet. Part of the settled sludge in the clarifier is recirculated to the aeration tank.

In most cases, the wastewater, besides containing organic pollutants, also has nitrogen, ammonia and phosphorous. These are called wastewater nutrients because they can cause excessive growth of algae (algae blooming) in the receiving water body. These nutrients are not removed in the aeration basin.

Some tertiary treatment using aerobic treatment systems with the application of zeolites also currently exist .

The zeolites are used in the form of powder, applied as in additive to water filtration and for wastewater treatment plants, as an inducer for sludge flocculation, sedimentation and nutrient reduction, together with the capacity to absorb recalcitrant substances. They are added to the influent stream or directly to the aeration basin.

There are several techniques that were developed to carry out the above mentioned treatments and some of them use zeolites. These include the systems of patents JP

10080697, KR 2005018777, RU 2060968; WO 85/04390; WO

02/044094.

The objective of patent JP 10080697 is to eliminate the need for a nitration stage requiring a long time, to regenerate zeolite and to rationally dispose of regenerated waste liquid by recycling the regenerated waste water from a zeolite regenerating stage to a biological denitrification part. To achieve this, a slurry is discharged from a denitrification tank and is aerobically treated in an aeration tank, the treated slurry is introduced into a settling tank to settle and separate the sludge, and the supernatant water is sent to a first zeolite-packed tank to remove ammonia. The water leaving

the first zeolite-packed tank is mostly drawn off as a treated water. Part of the treated water is introduced into a second zeolite-packed tank having adsorbed ammonia, air is blown into the tank, hence the water is biologically treated to regenerate zeolite, and the discharged water is joined to raw water as a biologically regenerated wastewater. The two zeolite-packed tanks are prepared in this way, the raw water is introduced into one zeolite- packed tank while the other zeolite-packed tank is biologically regenerated to adsorb and remove ammonia, and the process is alternately repeated.

Patent KR 2005018777 relates to a carrier comprising zeolite, scoria and ion charged minerals to biologically treat wastewater and wastewater treatment apparatus comprising a carrier disposed in a multistage aeration tank. The carrier has a granular structure constructed by bonding alkaline scoria usefully used as a carrier of living bodies, zeolite powder having high absorption capability and mineral powder charged with anions to prevent the acidification of the zeolite using cement and a wastewater treatment apparatus for effectively treating wastewater using the carrier. The wastewater treatment apparatus is characterized in that the wastewater is treated by continuously forming activated sludge layers at water inlet ports of aeration tanks and filling the carrier in the activated sludge layers.

Patent RU 2060968 relates to the treatment of organic sewage that comprises an installation with an aeration tank having a body with a rectangular cross section, bottom and aeration system, branch pipes supplying the untreated sewage water and discharging the unpurified sewage water and the charge in the form of a cassette . The cassette consists of screens arranged at a distance and zeolite tuff. The screens are made of an inert material with a layer of crushed zeolite tuff applied on them on both sides. The screens can also be made of pressed or monolithic zeolite tuff or in the form of two perforated walls with the crushed zeolite tuff placed between them. The unit is used in the aerobic purification of industrial wastewater ans sewage containing organic substances.

Patent WO 85/04390 relates to a process and equipment for removal of suspended material, biogenetic nutrients and dissolved metal compounds from sewage contaminated with organic and/or inorganic substances. In the course of the process contaminated water is contacted with an agent which contains salts or cations of mono, di or trivalent metals in a maximum amount of 50 Mp. cent and rock granules containing at least 25 Mp. cent zeolite, preferably clinoptlolite and/or mordenite . The amount of metal slats resp. cations present in the parent zeolitic rock can be increased according to necessity by ion- exchange, adsorption and/or impregnation, i.e. by combination of these processes following eventual

dehydration. The agent obtained is homogenized. It is favorable to carry out biological treatment without nitrification, up to an oxidation of carbon compounds. After secondary settling the water flows through one or more beds - if there are more, connected in series or parallel - filled with zeolite of a grain size between 1 and 10 mm, preferably between 2 and 5 mm, wherein residual ammonia is bound. The exhausted bed can be regenerated biologically or with an aqueous solution of potassium salt . The equipment consists of a primary settling tank, of an aeration basin and a secondary settling tank. The above agent from the preparation container and excess sludge from the secondary settling tank are admixed to the effluent of the primary settling tank in the mixing tank and the mixture is transferred in an adsorption zone communicating with the aerating basin. The rest of the ammonia and of the suspended materials of the effluent are bound and filtered out resp. in beds filled with zeolite in order to remove mechanical impurities. The beds are reflushed with purified water from a buffer tank and in order to reactivate the zeolite filling, aqueous potassium salt is fed from the container.

Patent WO 02/044094 relates to a process for enhancing the efficiency of sewage water purification and decreasing the reagent demand. This process is used to remove the dissolved and insoluble, organic and inorganic contaminating materials or combinations thereof from waters. The main feature of the process is that one or more

artificial or natural materials containing at least 1% weight crystalline alumino-hydrosilicate having a three dimensional crystal lattice comprising a pore structure of molecular size, preferably one or more types of rock granules containing clinoptilolite, mordenite and/or other zeolites are added to the water to be treated or to the activated sludge, and macromolecules comprising a large number of positively charged groups, preferably cation active polymers are bonded previously to the said materials by physical -chemical bonds so that free positively charged groups still remain on the bonded macromolecules. The bond between the solid material and the organic macromolecules is formed in a one or two step process.

None of the above mentioned patents can carry out secondary and tertiary treatment at the same time. Besides this, the zeolites are in modules inside a plug flow reactor. In this process the zeolites are confined and they are not lost with the excess biomass.

Treatment in separate stages automatically implies additional costs with regard to the treatment plant, which has to be equipped with separate basins or reactors for carrying out each stage, namely the second and third stages .

The loss of zeolites in the form of powder is another great inconvenience of the techniques existing in prior art .

The process of the present invention completely resolves these two problems .

In fact, the process of this invention carries out simultaneously the second and third stages, i.e. in the same reactor, and the zeolites are in modules inside a reactor. In this process, the zeolites are confined and they are not lost with the excess biomass.

Summary of the invention

The present invention has as its subject matter a process to be applied in the field of water treatment. It can be used to improve existing treatment plants or in new small, medium and large scale aerobic wastewater treatment plants, for either domestic or industrial effluents.

Contrary to what happens with the processes of the prior artr, this process can carry out at the same time secondary and tertiary treatment. Furthermore, it uses zeolites for the tertiary treatment and uses them without losses, i.e. the zeolites are confined in a special pack that the effluent crosses.

Zeolites are three-dimensional, microporous, crystalline solids with well-defined structures that contain aluminum, silica and oxygen in their regular framework; cations and water are located in the pores. The

silicon and aluminum atoms are tetrahedrally coordinated with each other through shared oxygen atoms. Zeolites are natural minerals that are mined in many parts of the world. Zeolites have void space (cavities or channels) that can host cations, water or other molecules.

Zeolites are natural minerals and they can be found in mines in several parts of the world.

The combination of various properties, including the microporous character and the uniform dimensions of pores, ion exchange capacity, the ability to develop internal acidity, high thermal stability and the high internal surface area, makes zeolites unique.

This process uses natural zeolites with the following characteristics:

Specific weight: 2.1 - 2.6 g/cm ³

- Porosity: 30 - 45%

Specific surface: 400 - 600 m ² /g

Stability in acid and alkaline media

Exchange capacity: 1.2 - 1.6 mol/Kg

Zeolites applied to wastewater treatment can increase biological activity, improve process efficiency, prevent bulking sludge, remove nutrients and reduce odours.

Brief description of the drawing

The description that follows is based on the attached drawing, which represents schematically the process sequence of the invention and also the constitution of the system.

Detailed description of the invention

As can be observed, the present invention refers to a wastewater treatment process using one or more stages

(6,11, ...) combined in series, each one including one chamber (4,9,...) for oxigen-enriched effluent aeration with biomass in suspension where secondary treatment takes place, followed by a chamber (5, 10,...) with immobilized zeolites where tertiary treatment takes place for additional biological conversion, of the biomass and specially for nutrient removal, and finally a module (12) for biomass retention in order to separate the suspended biomass and the clarified effluent.

The following are therefore fundamental characteristics of the invention: the use of one or more stages (6, 11) combined in series, each one including a chamber for aeration of the effluent with biomass in suspension (4, 9) , followed by a chamber with immobilized zeolites (5, 10) for improved biological conversion, retention of the biomass and removal of nutrients, and a module (12) for biomass retention in

order to separate the suspended biomass and the clarified effluent (13) ; and the secondary and tertiary processes are carried out at each stage.

The configuration of the reactor is of the plug flow type, where each pair of chambers, ordered from chamber (4) followed by chamber (5) , is combined together in one stage (6) and the several stages (11, ...) , being grouped in series to obtain the specific required treatment efficiency.

Each stage (6, 11...) is characterized by an aeration section or chamber (4) and a zeolite section or chamber (5, 10, ...) , thus in each stage suspended and attached biomasses coexist.

These combined chambers (4) and (5) will be differently aerated, thus different biomasses will develop according to the multi-parameters and operation conditions, such as different levels of oxygen, nitrogen and other nutrients in adsorption on the zeolites.

The chamber (4) can operates under very -high concentrations of dissolved oxygen, and with very high concentrations of biomass, due to the fact that the aeration process (3), for example using air ejectors, can supply up to 4.6 Kg O ₂ /Kwh, and the MLVSS (Mixed Liquor Volatile Suspended Solids) are kept above 10,000 mg/L,

because all the biomass existing at the final stage of the process is recirculated to the first stage.

The biomass filtration at the final stage in the module (12) is carried out using ultra- filtration membranes .

The chamber (5) consists of immobilized modules of zeolites, where the modules are box-like parts of adequate configuration, for instance a rectangular prism, containing macro-sized zeolites with a granulometry greater than 0.5 mm. The modules walls are made of a net with space smaller than the zeolite granulometry, to retain all the zeolites and at the same time allow the effluent to pass through the chamber (5) . The modules are packed in such a way as to avoid any preferred flow through the chamber (5) .

The fact that the zeolites are retained in the chambers (5, 10,...) prevents zeolite loss with the excess sludge produced in the process . Besides this and bearing in mind that the zeolites have to be renewed periodically, their retention in the chambers (5, 10, ...) allows them to be easily substituted simply by elevating these chambers in order to proceed with substitution.

The chambers (5, 10,...) operate under aerobic conditions in the outer layers in contact with the chambers

(4, 9 ,...), and under anoxic and anaerobic conditions in the inner layers. These conditions, combined with the

zeolites capacity to absorb and exchange ions and molecules, are used in the process to remove nitrogen and phosphorous from the effluent, and an additional absorption of recalcitrant substances and heavy metals is also achieved through the tertiary treatment.

In this reactor, the process combines secondary and tertiary treatment in one stage. This constitutes a great advantage as it makes it possible to eliminate tanks, reactors and lagoons after secondary treatment in order to proceed with the tertiary treatment, as happens in the treatment plants of the prior art.

When in operation, the reactor that uses the process of this invention is a combination of one or more stages in series, thus the effluent (1) is fed at stage (6) through the chambers (4) and (5) , passing from this stage, if necessary, to stage two (11) through chambers (9) and (10) and continuing through the following stages, in the quantity necessary to obtain final treated water with the required quality. At the outlet of the final stage (12), the effluent (13) is filtered in ultra-filtration membranes and discharged. The sludge retained in the membranes is totally recirculated to stage (6) . The excess biomass produced in the chambers (4, 9, ...) is eliminated in view of the oxygen levels in the various chambers, by the cellular retention time in the zeolite chambers and by the biological conversion which occurs in the chambers (5. 10, ...) . The process operates continuously.

The back flush of the biomass fixed in the chambers (5, 10,...) is ensured by discontinuous effluent circulation from chamber (4) to the chamber (9) , by pumping with the pump (15) the effluent from one aeration chamber

(for instance chamber 4) to the adjacent aeration chamber

(for instance chamber 9) through a bypass pipe (14) . This creates a different liquid level between the adjacent chambers, which produces a counterflow of the effluent through the zeolite chambers (for instance 5) with the removal of biomass existing in the said zeolite chambers

(for instance 5) .

As can be inferred from the above description, the main advantages of the present invention when compared to current wastewater treatment processes are:

oxygen enriched aeration (4.6 Kg O ₂ /Kwh) combined with zeolites resulting in increased biochemical activity of the activated sludge; the concentration of the MLVSS (Mixed Liquor Volatile Suspended Solids) is kept above 10,000 mg/L, because all the biomass existing in the final stage of the process is recirculated to the first stage; the bacteria are accessible in high concentrations for the utilization of substrate and oxygen; the sludge activity per unit volume is increased; operating load: 10 - 50 Kg of BOD ₅ /m ³ process volume; secondary and tertiary treatment together in one

st age ; and zeolites are not released and discharged together with the excess sludge produced in the process. They are confined in their own chamber.