CA3074222A1

CA3074222A1 - System and method for wastewater treatment

Info

Publication number: CA3074222A1
Application number: CA3074222A
Authority: CA
Inventors: Scott Robinson
Original assignee: Unit Precast Breslau Ltd
Current assignee: Unit Precast Breslau Ltd
Priority date: 2020-02-28
Filing date: 2020-02-28
Publication date: 2021-08-28
Also published as: CA3076070A1

Abstract

Wastewater from a first moving bed biofilm reactor (MBBR), allocated for organic carbon removal, is directed to an Intermediate Clarifier (IC) to separate solids/biomass, before entering the second MBBR Bioreactor (designed for nitrification).

Description

SYSTEM AND METHOD FOR WASTEWATER TREATMENT
FIELD OF THE INVENTION
[0001] The present invention relates to the field of wastewater treatment using moving bed biofilm reactor (MBBR) process technology, in which effluent quality is improved by including one or more Intermediate Clarifiers (IC) downstream from one or more MBBRs.
BACKGROUND OF THE INVENTION

[0002] Wastewater may contain contaminants and microorganisms that threaten human health and the ecosystem. Although the environment naturally treats wastewater, such process is slow enough to damage the ecosystem and imbalance species.

[0003] A moving bed biofilm reactor (MBBR) system of the prior art is illustrated in Fig. 1.
The MBBR, inspired by the principles of natural treatment processes, is a compact attached growth and is tremendously efficient technology for organic carbon removal, nitrification (ammonia removal), denitrification (nitrate/nitrite removal) and other biological treatment processes. MBBR
technology was originally invented by a Norwegian Professor, HaIlvard Odegaard (Norwegian Patent No. 02074470), and has been successfully implemented in many installations in more than 50 countries. Benefits of MBBR technology include: high-efficiency, easy operation, independence of biomass/sludge return, fluidization approach (to eliminate risks associated with media clogging), a small footprint compared to conventional wastewater treatment systems, easy capacity upgrade (to respond to an increase in a population without expanding current existing footprint), and resistance to hydraulic shock thanks to its biological attached growth system.

[0004] MBBR technology has been adapted and used for treatment of domestic, commercial, and industrial wastewater with various water characteristics. High strength wastewater is typically generated from commercial establishments, funeral homes, food processing facilities, nursing homes, restaurants, spas, and other similar facilities that generate wastewater different from residential wastewater in terms of quantity and quality. According to Wastewater Engineering Handbook (Metcalf & Eddy, 2013, page 221), high strength wastewater includes any wastewater with organic carbon (measured as Carbonaceous Biochemical Oxygen Demand, CBOD) equal or higher than 400 mg/L.

[0005] The objective of any biological treatment is to transform soluble organic materials and nutrients (nitrogen and phosphorous) in wastewater into biofilm and biological flocs, which are settleable biomass/solids. In the known MBBR technology, a variety of microorganisms in the form of attached biofilm grow on the surface of MBBR media. MBBR media typically provide a large surface area from 300 m2/m3 to 1200 m2/m3 biofilm growth. In the prior art, aeration is used to fluidize MBBR media and provide oxygen for oxidation of organic materials and nutrients by microorganisms. As the microorganisms grow, the biofilm thickens, and eventually excess biomass sloughs/detaches from the surface of the media and forms biological suspended solids/biomass.

[0006] The main objective of biological wastewater treatment in an aerobic MBBR system is to remove contaminants in wastewater (e.g., organic carbon and ammonia in this disclosure) by utilizing microorganisms as cleaning agents. In the known MBBR systems, microorganisms consume soluble wastewater contaminants and transform them into biomass/solids. The characteristics of the target contaminant (e.g., organic carbon or ammonia) dictate the stoichiometry of the biological processes occurring in the MBBR system and amount of produced biomass.

[0007] A typical MBBR treatment system of the prior art is illustrated in Fig. 1. In the first MBBR bioreactor shown in Fig. 1, removal of soluble/particulate organic carbon, measured as CBOD (Carbonaceous Biochemical Oxygen Demand [mg/L]), takes place; and the second MBBR
bioreactor shown in Fig. 1 is allocated for the oxidation of ammonia to nitrate and nitrite. The latter process is called nitrification. Certain groups of microorganisms contribute in these bioprocesses of contaminant removal.

[0008] Organic carbon removal microorganisms (heterotrophs) use carbonaceous organic materials as their food source (substrate) in general accordance with the following expression:
Bacteria Oragnic matter + 02 + Nutrients -- CO2 + New biomass + Other products , ,

[0009] The substrate for nitrifying bacteria is ammonia:
Nitrif iers Ammonia + 02 +Alkalinity + Nutrients - NO3 + New biomass

[0010] The ratio of biomass produced to the amount of substrate consumed is called yield:
New biomass Yield =
Consumed substrate

[0011] In other words, for the organic carbon removal, process yield indicates the mass of biomass to the mass of organic carbon consumed; and in nitrification, yield indicates the mass of biomass to the mass of ammonia oxidized. Higher yield value indicates a higher amount of biomass produced through the treatment process. The typical value for organic carbon removal yield is 0.6 g VSS/ g BOD, while the typical value for nitrification yield is substantially smaller (as low as 0.12 g VSS/ g NH4-N).

[0012] The kinetic and reaction stoichiometry suggests that, depending on the organic carbon concentration, most biomass in biological treatment systems is probably produced in the first MBBR. The amount of biomass produced in the second MBBR (nitrification) would be even less than the predicted stoichiometry values, because a portion of nitrogen will be consumed through the organic carbon removal process. In general, for every 100mg of organic carbon removed in the first MBBR, 5mg of nitrogen (in the form of ammonia, NH3-N) will be used for biomass/solids production. The remaining ammonia will be oxidized to nitrate and nitrite in the second MBBR through the nitrification process, producing a small quantity of solids/biomass (depending on the concentration of oxidized ammonia).

[0013] In conventional MBBR systems (Fig. 1), the biomass produced though the organic carbon removal process typically travels through the second MBBR to reach the secondary clarifier. The amount of biomass produced in the first bioreactor depends on the organic carbon strength in wastewater, and increases as the concentration of organic carbon in raw wastewater increases. For most of the suspended growth or attached growth (with fixed media) wastewater treatment technologies, this would not be problematic as there are no moving objects (media) in those systems. However, the MBBR process is different due to the presence of moving media.
Biomass structure, its floc formation, and settling properties of sludge can adversely be influenced when traveling through the second MBBR bioreactor. High-shear stress and collision of MBBR
media inside the MBBR bioreactors probably contribute in the destruction of the biomass settling property.

[0014] As is known in the art, the detached biomass/solids should be separated from water, to provide a clear effluent with an acceptable TSS concentration. The specific gravity of the biomass in most biological treatment technologies is slightly greater than the specific gravity of water, making the "gravity-based clarifier' a successful method for separation of biomass/solids from water for some technologies. However, biomass from the typical MBBR
process has poor settling properties, causing challenges in complying with regulatory TSS
targets.

[0015] Investigations have been conducted to understand the phenomena behind this issue and how to address it. Most of the known proposed solutions offer implementation of high-efficiency and expensive solid-water separation technologies downstream of MBBR treatment systems. These solid-water separation technologies include physical separation methods (such as dissolved air flotation, membrane, filtration, lamella, etc.), chemical separation methods (such as the addition of coagulants or coagulant-flocculant chemicals), or combinations thereof.
However, all of the known proposed solutions aim to improve the separation efficiency of biomass, with little attention to the root cause of the poor settling property of MBBR
biomass in gravity settling clarifiers.
SUMMARY OF THE INVENTION

[0016] The system and method of the invention reduce the TSS
concentration in treated effluent from MBBR wastewater treatment systems by enhancing the settleability of biomass/solids. The system and method may be used in the treatment of high-strength wastewater using the MBBR systems and in the treatment of low-strength wastewater.

[0017] It is believed that aggressive agitation, high shear stress, and free movement/floating of MBBR media in bioreactors potentially contribute in damaging the biomass structure and reducing its settling property (settleability). Therefore, higher retention time in bioreactor(s) worsens the settleability of biomass, making TSS separation a considerable challenge in the clarifier.

[0018] In the system of the invention an intermediate clarifier (IC) is positioned immediately downstream from the first MBBR bioreactor (allocated for organic carbon removal) where the biomass is fresh and meets its highest production rate.

[0019] To achieve high biomass/solids separation in IC, the first MBBR
preferably is designed and sized to ensure a complete (or substantially complete) organic carbon removal, with a minimal possibility of nitrification process occurrence (oxidation of ammonia to nitrate). A
partitioning of the MBBR bioreactor into two or more compartments is believed to assist in maintaining high organic carbon removal performance, even during high organic carbon loadings and hydraulic shocks.

[0020] Nitrate and nitrite from the nitrification process may also interfere with the proper settling of biomass in the IC. Lack of dissolved oxygen and the presence of nitrate and nitrite may trigger the denitrification process at the bottom of hoppers, generating nitrogen gas in the settled sludge blanket. The nitrogen bubbles that are trapped in the sludge flocs may resuspend the settled sludge and reduce the TSS removal efficiency in the IC.

[0021] A wide range of technologies for solids-water separation may be employed as an IC, including, for example: conventional gravity-settling clarification, dissolved air flotation, lamella clarification, chemically-enhanced clarification, membrane, and filtration. In the case of using a conventional gravity-settling clarifier as an IC, the components such as hoppers, the weir, and sludge and scum pumps are required to be installed for collecting and transferring the separated biomass/solids to the sludge storage facility. The sizing of the sludge and scum transferring equipment may be determined based on the amount of produced biomass in the organic carbon removal bioreactor(s).
BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention will be better understood with reference to the attached drawings, in which:

[0023] Fig. 1 (previously described) is a schematic cross-sectioned side view of a conventional MBBR treatment system, consisting of the first MBBR (allocated for organic carbon removal), followed by the second MBBR (allocated for nitrification) and a Secondary Clarifier;

[0024] Fig. 2 is a schematic cross-sectioned side view of a MBBR
process, equipped with the Intermediate Clarifier (IC), installed immediately after the first MBBR
(organic carbon removal) and prior to the second MBBR (nitrification);

[0025] Fig. 3 is a schematic cross-sectioned side view of full-mixed versus partitioned MBBR (three compartments); and

[0026] Fig. 4 is an illustration of CBOD5, BOD5, and TSS concentrations over time in effluent of a high-strength wastewater MBBR treatment system, consisting of three stages: (1) before the addition of an IC (having three MBBRs in series for organic carbon removal and nitrification), (2) turning off the aeration in the middle MBBR, (3) retrofitting the middle MBBR to an IC.
LIST OF REFERENCE CHARACTERS
1. Influent pipe, inlet wastewater pipe 2. Backflow valve, to prevent MBBR media traveling to the prior tank(s) in the case of back flow 3. Aeration pipes 4. Aeration diffusers 5. Access riser opening(s), only for roof-tank(s) (close-top tank(s)) 6. Slotted/Screen pipes, to screen out and prevent media travel to the subsequent compartment(s)/tank(s) 7. Partition wall(s), to divide/split a MBBR bioreactor into smaller compartments (2, 3, or more compartments) 8. Sludge pump(s), to collect and transfer the sludge from clarifier(s) to the sludge storage facility 9. Scum pump, to collect and transfer scum (floating sludge accumulated on the surface of water) from clarifier(s) to sludge storage facility 10. Sludge transfer pipe, to transfer sludge to sludge storage facility 11. Weir, to collect treated effluent from clarifier(s) DETAILED DESCRIPTION

[0027] In one embodiment, the system of the invention preferably includes an IC to settle and separate the biomass immediately after the organic carbon removal MBBR, where the biomass reaches its highest production rate and its structure is still generally untouched, and intact (Fig. 2). Fresh biomass in effluent of organic carbon removal MBBR has superior floc formation and settling properties and can be removed effectively and efficiently by any gravity-based clarifier. Gravity-based clarifiers are technologically simple, cost-effective, and easy to operate.

[0028] The small quantity of biomass generated in the nitrification MBBR
will be settled and separated in a secondary clarifier, installed after the second MBBR (Fig.
2).

[0029] To improve organic carbon removal performance, specifically in the course of high-loading or hydraulic shocks, in one embodiment, the MBBR(s) preferably are partitioned (Fig. 3).
Despite having the same effective volume, a partitioned MBBR maintains a high concentration of organic carbon in the early compartments, boosting the removal rate (kg BOD/m2.d). In addition, partitioning of the organic carbon MBBR prevents short-circulation of raw wastewater to the effluent stream of the bioreactor without the proper treatment. The partitioning approach can be used for the MBBR nitrification process as well.

[0030] Organic carbon removal process is the main source of biomass production in MBBR systems, and it is important to ensure a full (or substantially full) organic carbon conversion to biomass. However, an aggressive design of large MBBR bioreactor(s) may also lead to an over-sizing issue which may cause occurrence of the nitrification process (nitrate/nitrite production) in the organic carbon removal MBBR. Nitrate/nitrite from the nitrification process may interfere with proper sludge settling in the IC. The concentration of Dissolved Oxygen (DO) is usually low in clarifiers, creating an anoxic zone, which is a suitable environment for the denitrification process. Denitrifying bacteria in settled sludge utilize nitrate/nitrite as election acceptor (replacing oxygen) and generate nitrogen gas:
denitrif iers Oragnic matter + Nitrate or Nitrite + Nutrients N2(gas) + New biomass

[0031] The generated nitrogen gas may dissolve in water or may appear in the form of small bubbles trapped in settled sludge at the bottom of hoppers. These small nitrogen bubbles may disturb the settled sludge blanket at the bottom of the IC, floating a portion of biomass flocs to the surface of water in the form of floating scum. In this situation, a scum pump would be necessary to collect and transfer scum from the water surface to sludge storage facilities.

[0032] A disturbance of the sludge blanket may lead to incomplete separation of biomass and an increase in effluent TSS. To prevent oversizing of the organic carbon removal MBBR, it is preferred that a surface area loading rate of 3 g BOD/m2.d or larger for all compartments is maintained.

[0033] The amount of settled biomass in either the intermediate or secondary clarifier is a function of biomass produced in the bioreactor(s) prior to (i.e., upstream in relation to) these clarifiers. The biomass amount can be calculated by available simulation software, or use of kinetic and reaction stoichiometry of organic carbon removal and nitrification reactions.
Determining the amount of biomass will assist with sizing the sludge collection and return equipment in the selected solids-water separation technology.

[0034] A variety of pumps, such as centrifugal, air lift, screw, positive displacement, and so on, can be employed for transferring sludge and scum.

[0035] In the case of using the conventional gravity-based IC, it is believed that the overflow rate shall be in the range of 0.5 to 16 m3/m2.d; however, in tests, the highest solids separation performance was observed in the range of 0.5 to 8 m3/m2.d.

[0036] Those skilled in the art would appreciate that microorganisms taking care of wastewater treatment in MBBR systems are immobilized/attached on the surface of moving media and are not washed out with effluent flow. This distinguishes the MBBR system from the suspended growth treatment technologies, such as activated sludge or extended aeration, in which microorganisms are suspended and travel with effluent flow. Because of the attached growth nature of microorganisms in MBBR systems, TSS in effluent is substantially smaller than the effluent of those suspended growth technologies. Due to small concentration of solids in MBBR systems, the solids loading rate preferably is not used as a design parameter for sizing of either the intermediate or secondary clarifier.

[0037] The shape of the IC tank can be either rectangular or circular in plan view, preferably with a minimum hydraulic retention time (HRT) of 3 hours. The HRT
and overflow rate are influenced by the additional flow resulting from internal recirculation (IR) flow in the case of performing the pre-denitrification process. In the pre-denitrification process, a portion of water flow is returned/recirculated from effluent of nitrification bioreactor(s) to the primary stage of the treatment plant, to be combined with raw wastewater. The IR flow reduces the actual HRT and overflow rate values in the IC, and therefore the actual flow should be taken into consideration for sizing the IC.

[0038] Hoppers, weir, sludge, and scum pump(s) preferably are used for biomass collection from the bottom, as well as the surface of the IC. Access openings and risers preferably are included, for servicing equipment and pump(s) in close-top tank clarifier(s). The diameter of access opening depends on the size of equipment installed inside the IC and the available access risers. Guide rails can be utilized for lifting and servicing equipment as heavy as 50 pounds or more.

Claims

We Claim:

1. A system for treatment of wastewater, the system comprising:
a first moving bed biofilm bioreactor, for at least partially converting organic carbon in the wastewater to biomass to provide first stage treated wastewater comprising solids and water, the first moving bed biofilm bioreactor being divided by at least one internal partition into internal compartments through which the wastewater passes;
an interim clarifier located downstream from the first moving bed biofilm bioreactor, for at least partially separating the solids in the first stage treated wastewater from the water thereof, to provide second stage treated wastewater comprising solids and water; and a second moving bed biofilm bioreactor, for at least partial nitrification of the solids in the second stage treated wastewater, to provide third stage treated wastewater comprising solids and water.

2. A system according to claim 1 additionally comprising a secondary clarifier located downstream from the second moving bed biofilm bioreactor, for at least partially separating the solids in the third stage treated wastewater from the water thereof.

3. A system for treatment of wastewater, the system comprising:
a first moving bed biofilm bioreactor, for at least partially converting organic carbon in the wastewater to biomass to provide first stage treated wastewater comprising solids and water;
an interim clarifier located downstream from the first moving bed biofilm bioreactor, for at least partially separating the solids in the first stage treated wastewater from the water thereof, to provide second stage treated wastewater comprising solids and water; and a second moving bed biofilm bioreactor, for at least partial nitrification of the solids in the second stage treated wastewater, to provide third stage treated wastewater comprising solids and water, the second moving bed biofilm bioreactor being divided by at least one internal partition into internal compartments through which the second stage treated wastewater passes.

4.
A system according to claim 3 additionally comprising a secondary clarifier located downstream from the second moving bed biofilm bioreactor, for at least partially separating the solids in the third stage treated wastewater from the water thereof.