Moving Bed Biofilm Reactor (MBBR) Process
The MBBR process is a type of attached-growth process. In MBBR reactors, microorganisms are fixed on moving carriers for carbon removal and/or nitrification. The plastic media carriers are specially designed to provide a favorable environment for sustained and stable microbial growth, with a density slightly lower than water.
The MBBR process operates with continuous aeration. The purpose of air supply is to provide oxygen required by microorganisms and to ensure the suspension of the media carriers.
The effluent from the reactor, along with the sludge mixture, is sent to downstream treatment units or sedimentation tanks. A certain proportion of settled sludge is returned to the inlet of the MBBR process. Static screens with bar spacing matching the size of the media carriers are used to retain the carriers within the reactor, as shown in the diagram below:
The supporting media comes in two forms, as shown in the diagram: a. MBBR C, used for carbonaceous pollutant treatment; b. MBBR N, is primarily used for nitrification treatment.
Effective pretreatment, such as a primary settling tank or fine screen, is required before the influent enters the MBBR reactor.
1.MBBR C
MBBR C uses polyethylene material as circular media carriers with a diameter of 45mm and a length of 35mm. The specific surface area is approximately 310m²/m³, and the bar spacing of the static screen is 25mm.
MBBR C is a high-load biological process (volumetric load up to 30kg COD/(m³·d)), mainly used for industrial wastewater treatment, with a maximum media filling ratio of 40% of the reactor volume.
Depending on the treatment objectives, this process can be used for:
①Pre-treatment upstream of activated sludge processes, often applied in upgraded wastewater treatment plants.
②Secondary sedimentation processes in two-stage treatment or flotation processes for certain industrial wastewater.
The two schemes are shown in the diagram:
2.MBBR N
MBBR N also uses polyethylene material as circular media carriers, but smaller than those used in MBBR C: diameter of 10mm and length of 7mm. The effective specific surface area is larger, approximately 870m²/m³. The bar spacing of the static screen is 5mm.
MBBR N has two different applications:
① Nitrification reaction of pretreated water in the MBBR reactor, followed by denitrification reaction in the anoxic zone through mixed liquid recirculation and sludge recirculation (referred to as MBBR CN). This system is similar to traditional activated sludge systems aiming for integrated denitrification, with the main difference being that the minimum sludge age is no longer a limiting parameter for the design volume of aerobic tanks, resulting in a significant reduction in required aerobic tank volume (reduced to approximately 1/3). Additionally, due to the shorter sludge age and a diverse population of heterotrophic bacteria, higher denitrification rates can be achieved. MBBR CN is particularly suitable for:
a.Upgrading wastewater treatment plants without major modifications: Existing reactors can be divided into two independent zones (anoxic and MBBR CN) to enhance the feeding capacity of the aeration system. The installation of mixed liquid recirculation pumps and media-retaining static screens allows a wastewater treatment plant originally designed for carbonaceous pollutant removal only to achieve TN discharge standards.
b.Small-scale wastewater treatment plants located in cold regions or experiencing seasonal fluctuations in pollution loads: Bacteria attached to the carriers ensure a quick start-up after shutdown and maintain nitrification even under low-temperature conditions.
② Deep nitrification treatment is applied to the effluent from the secondary treatment, followed by solid-liquid separation (MBBR N). The primary objective is to remove ammonia nitrogen or meet TKN (Total Kjeldahl Nitrogen) discharge standards. The MBBR N process is also suitable for high-concentration ammonia nitrogen wastewater, such as the nitrification treatment of the sludge return from the nitrification tank.
The two application schemes are illustrated in the following diagram:
3.Advantages and Disadvantages
The MBBR process offers the following key advantages:
① Rapid and efficient removal of carbonaceous pollutants [influent volumetric load of up to 30kg COD/(m³·d)] and ammonia nitrogen [volumetric load of up to 0.6kg NH+ 4-N/(m³·d)].
②Significant reduction in reactor volume.
③Stable treatment performance with strong resistance to shock loads.
④ Suitable for wastewater treatment plant upgrades and expansions.
However, the MBBR process has the following disadvantages:
① It may be challenging to achieve thorough removal of total nitrogen for wastewater with low BOD/TKN ratio and high TKN concentration.
② The nature of the residual sludge generated in medium-load biochemical treatment processes may be unstable.