Submerge and Conquer

With the help of submerged membrane modules, a malting company in Antwerp overcomes the disadvantages of membrane bioreactor technology use in large-scale applications, becoming the largest industrial membrane bioreactor plant in Belgium

Since the late 1990s membrane bioreactor (MBR) technology has rapidly entered the wastewater treatment market. The technology is a simple combination of an activated sludge process and a membrane filtration step. The separation of activated sludge and effluent is achieved by using porous membranes that are able to remove all the suspended solids from the biologically cleaned water. The principle of this technology is not new since membrane bioreactor technology with external pressurized membrane modules has been used in industrial applications for more than 25 years. However, the biggest disadvantages of this technology for large-scale applications are the high investment costs and, especially, the high energy consumption due to the fact that the external membranes have to be operated in cross flow mode using high feedside velocities. An option for eliminating both of these disadvantages is the introduction of submerged membrane modules into the membrane bioreactor technology.

Membrane bioreactors using submerged membrane modules are being increasingly used in industrial wastewater treatment applications. The use of MBR technology in wastewater treatment plants improves water quality considerably and requires less space compared to conventional methods. The recycling of process water can help industrial companies to cut wastewater disposal costs and to reduce consumption of fresh water. The largest industrial membrane bioreactor plant in Belgium was commissioned in November 2004. The wastewater treatment plant is equipped with sixteen PURON® membrane modules. The submerged membrane filters, with a total membrane area of 8,000 square meters (m2), are treating the wastewater of a malting company in Antwerp.

The Malting Company
The Belgian malting company Sobelgra is located in the Antwerp harbor and is part of the multinational Boortmalt group. The company produces malt for breweries. Malt is the basic ingredient used in the production of beer. The main raw material used in the production of malt is barley. After a thorough cleaning and removal of impurities, chaff, and broken or low-grade kernels, the barley is germinated by soaking it in water over several days. Once enough enzymes have been formed, the process is stopped by means of heating. The color of the beer is influenced by this drying process as well. The entire malting process requires a tremendous amount of fresh air and fresh water.

Sobelgra is currently extending its production from 110.000 to 250.000 metric tons per year. The plant will then be the largest independent malting company in Belgium. The capacity of the existing conventional wastewater treatment plant had to be doubled as well. Due to lack of space on the factory site, conventional wastewater technology could not be used. The compactness of membrane bioreactors was the main reason why Solbregra selected this innovative technology. The capacity of an existing wastewater treatment plant can be enlarged without increasing floorspace since the higher bacteria concentration of the sludge increases the performance of the biological step. Additionally, clarification tanks become unnecessary since the separation of sludge and clean water is done by the membranes. The existing infrastructure could therefore be used for the extension of the wastewater treatment plant; there was no need to build new tanks. This meant that separation walls were inserted into the existing biological tank. One half now serves as a membrane tank for the submerged modules. The former sedimentation tank of the clarifier is now part of the biological treatment process.

In spring 2003, the biological process parameters and the most suitable membrane technology were determined during an on-site pilot study that lasted several months. The submerged PURON hollow-fiber modules were then selected. The Belgian turnkey constructor ENPROTECH was the general contractor for this project and supplied the process design, civils, electro-mechanical, automation, visualisation, and electrical equipment. The subcontractor for the supply and assembly of the filtration system was the Belgian Engineering Company.

The Wastewater Treatment Plant
The wastewater treatment plant consists of three stages:

  • A mechanical pretreatment,
  • A biological stage, and
  • The membrane filtration system.

After coarse impurities from the barley processing have been removed by two curved sieves in the mechanical pretreatment stage, the wastewater is fed into the biological stage. The two curved sieves have a mesh size of 0.25 millimeters (mm). The biological stage consists of two aeration tanks connected in series. After a sufficient retention time in the biological stage, the treated water is fed into the membrane stage where it is separated from the activated sludge. The membranes form an absolute barrier to suspended solids and microorganisms. The membrane stage consists of three chambers into which the PURON modules are submerged. In the first phase of expansion, two chambers are equipped with eight modules each. The third chamber is available for future expansions of the plant. The chambers are fed with activated sludge from below so that the sludge flows through the modules from bottom to top. The clean water is sucked out of the membrane modules by means of a vacuum. The concentrated activated sludge is led via spillways back into the aeration tanks. In order to maintain the filtration rate of the membrane modules, a backwash combined with air scouring is carried out at regular intervals. The chambers can be decoupled independently for cleaning and maintenance purposes.

The control equipment (blowers, pumps, electrical equipment, etc.) is located in a control room next to the tanks. Implementing MBR treatment has allowed the malting company to successfully expand without taking up further space on their premises. At the same time, discharge quality of the effluent has increased considerably compared to the conventional wastewater treatment plant. Table 1 shows some of the plant's effluent requirements.

The MBR plant commenced operation in November 2004. During the commissioning phase from November 2004 to January 2005, only one third of the biological plant was in operation. Permeability was high at 530 liters per square meter per hour (l/m2h) bar (a unit of pressure equal to 100,000 pascals or to one million dynes per square centimeter or to 0.9869 atmosphere) in November and rose to 610 in January, as the biological process has started to be optimized. The effluent COD (chemical oxygen demand) values have been at or below the plant's requirements. The flow rate rose from 20.2 cubic meters per hour (m3/h) to 35 m3/h. Once the plant is operating at full capacity with both aeration tanks, more than 2,000 cubic meters (m3) of wastewater will be treated per day. Table 2 shows some parameters during the commissioning phase.

The Membrane Filter Modules
At the heart of the wastewater treatment plant are the submerged hollow-fiber membrane modules. The modules are especially designed for the extremely tough requirements in wastewater -- and here particularly for biological wastewater treatment in membrane bioreactors. A key factor for a stable and reliable operation of MBR plants with high flow rates is effective "solids management" in the membrane modules, i.e. the reliable removal of all filtered substances out of the system. The PURON system features hollow-fiber membranes that are fixed only at their lower end. They are operated on the outside-to-inside principle, i.e. all solids and particulates remain on the outside of the membranes while permeate is withdrawn from the inside of the fibers. The membrane pore size is between 0.1 micrometers (ìm) and 0.2 ìm. The lower ends of the membrane fibers are fixed in a header while the upper ends are individually sealed and are free to move laterally. A braid inside the membrane material provides enough mechanical strength to ensure that the fibers cannot break during operation. An air nozzle is integrated into the center of each fiber bundle to apply the air for scouring purposes. The fiber bundles are connected to rows. Several of the rows are mounted into a common steel frame and form the membrane module. The filtrate is removed out of the system via the header and the lateral tubes. The header allows for both collection of the filtrate and distribution of scouring air inside the module. The central arrangement of the air nozzles inside the membrane bundles reduces the energy need for module aeration. The lower air consumption allows for the installation of smaller aeration blowers. In order to prevent the membrane fibers from tipping over during insertion or removal, fiber supports are fixed to the lateral filtrate removal pipes. These supports provide enough space for the membrane fibers to move freely.

MBR Applications
Membrane bioreactors using submerged membrane modules are increasingly applied in industrial wastewater treatment since this technology offers many advantages for industrial companies. It may help to close water cycles, for example, by reusing the treated wastewater as process water. The costs of wastewater disposal can be reduced considerably while saving fresh water at the same time. Examples for industrial applications include: food and beverages, textiles, pulp and paper production, laundries, etc. This technology is also interesting for countries with water shortages where the effluent of membrane bioreactor plants can be used for irrigation purposes, process water applications, or as part of the treatment process for indirect potable reuse. The effluent quality is far beyond the current regulatory requirements for discharge into the environment. It even meets the stringent requirements of the European bathing water directive. In general, MBR technology is always attractive for applications where:

  • A compact technology is required because of lack of space or the high cost of land in urban areas, and
  • When a high effluent quality is needed (irrigation, golf courses, industrial use, pretreatment before nanofiltration, and reverse osmosis)

Table 1: Plant Requirements

Parameter

Influent mg/l*

Load kg/day**

Effluent

mg/l

COD

(Chemical Oxygen Demand)

1880 - 2100

4000

100-200

BOD

(Biological Oxygen Demand)

700 - 930

2000

2-5

Suspended solids

330 - 460

800

0

Total-Nitrogen

35 - 50

100

1-2

Total-Phosphorous

13 -15

30

< 1

* Milligrams per liter

** Kilograms per day


Table 2: Commissioning Phase

Parameter

Nov. 04

Dec. 04

Jan. 05

Permeability

l/m2h bar*

530

520

614

MLSS g/l

5

14.67

10.96

Flow m3/h

20.2

32.5

35

COD effluent mg/l

131

78

133

* Gallons per liter

This article originally appeared in the 03/01/2005 issue of Environmental Protection.

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