A Key Component of IMO-Compliant Marine Sewage Treatment Plant: Membrane Bioreactor (MBR) Technology

As the global shipping industry faces increasingly stringent requirements for green development and environmental compliance, the International Maritime Organization (IMO) and various flag states have tightened regulations on the discharge of shipboard sewage. Traditional sewage treatment methods are finding it difficult to meet the practical needs of modern vessels, particularly regarding treatment precision, space requirements, operational maintenance, and long-term stability. Marine sewage treatment systems are essential equipment for ensuring regulatory compliance and protecting the marine environment; the effectiveness of the biological treatment process within these systems directly determines both the quality of the effluent and the overall operational efficiency.

Among the various available sewage treatment technologies, the Membrane Bio-Reactor (MBR)—which integrates membrane separation technology with the activated sludge process—has emerged as a highly advanced and representative solution for treating shipboard sewage. Within an MBR system, the biological membrane serves as the core medium for pollutant degradation, while also playing a critical role in achieving efficient solid-liquid separation and ensuring stable effluent quality throughout the treatment process.

Introduction to Membrane Technology

1. Classification of Membranes

Based on pore size, membranes are generally categorized into the following four types: Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF), and Microfiltration (MF).

⑴ Reverse Osmosis (RO):

Capable of retaining all dissolved salts and organic substances with a molecular weight greater than 100 Daltons.

⑵ Nanofiltration (NF):

Its filtration performance falls between that of ultrafiltration and reverse osmosis; it can retain particles with a diameter greater than 0.001 μm (1 nm).

⑶ Ultrafiltration (UF):

Capable of retaining particles with diameters ranging from 0.002 to 0.1 μm. It is typically used for reverse osmosis pretreatment, the concentration of macromolecules, fractional concentration, and the purification of salts. (Our marine sewage treatment units utilize this type of membrane.)

⑷ Microfiltration (MF): Capable of retaining particles with diameters ranging from 0.1 to 10 μm.

2. Classification of Ultrafiltration Membranes

• Submerged (internal) membranes

• External membranes

3. Classification of MBR Processes

A membrane bioreactor (MBR) primarily consists of two components: a membrane module and a bioreactor. Based on how these two are integrated, MBRs can be categorized into three types: external MBRs, internal (submerged) MBRs, and hybrid MBRs.

(1) External MBR

In this configuration, the membrane module and the bioreactor are set up separately and function independently, connected by pumps and piping. Because the components operate separately, there is minimal mutual interference, making the system easier to regulate and control. Furthermore, since the membrane module is located outside the bioreactor, it is more accessible for cleaning or replacement. However, this type has higher energy consumption; a high-pressure pump is required to generate a high-velocity cross-flow across the membrane surface, which helps delay membrane fouling.

(2) Internal MBR

The membrane module is placed directly inside the bioreactor—a configuration also known as a submerged membrane bioreactor (SMBR). Permeate (treated water) is extracted using gravity, suction from a pump, or a vacuum pump. Because the membrane module is submerged in the mixed liquor of the bioreactor, it is prone to rapid fouling. Additionally, cleaning is relatively cumbersome, as the membrane module must be removed from the reactor.

(3) Hybrid MBR

In a hybrid MBR, the membrane module can be positioned either inside or outside the bioreactor, with permeate extracted via suction or pressure. What distinguishes this type is the inclusion of packing material (media) within the bioreactor, creating a hybrid treatment system.

Given the complex operating conditions on board a ship—where removing membranes for cleaning or replacement is highly inconvenient—this system utilizes an internal (submerged) membrane configuration with an in-situ (online) cleaning method. Much like the operating principle of a washing machine, this design eliminates the need for manual removal and cleaning, representing a highly innovative approach.