Carrier-based Anaerobic Treatment
Carrier-based Anaerobic Wastewater Treatment – a Unique Solution
COMMITTED TO SUSTAINABILITY: The impact of environmental matters on business performance in the food and beverage industry is increasing and will continue to do so. In the dynamic field of food and beverage wastewater treatment increasing numbers of new systems have been tested over the past decades. Aqana – Aqwise, an Israeli-Dutch joint venture has been increasingly successful with a reliable, ingeniously simple, very efficient, and robust solution for handling these industries’ effluents.
The latest technologies aim to meet ever stricter regulative demands as well as to ensure continuous improvement towards operability and efficiency, thus reducing operational expenditure. By being able to set precise environmental Key Performance Indicator (KPI) targets – commercial as well as social – KPIs are being taken to the next level.
One of the main challenges of conventional anaerobic wastewater treatment, its operational continuity, has been largely overcome by the elimination of the necessity of granular sludge. The direct result is a system that is significantly more stable because it is less sensitive to flow, load, and effluent quality fluctuations. This newly developed and patented technology, derived from established in-house MBBR technology, is based on an anaerobic fluidized carrier bed – Downflow Anaerobic Carrier System or DACS.
Industrial Wastewater Treatment
With the growing industrial activities in the last century, the discharge of pollutants into natural ecosystems increased significantly. The loading rates to these receiving waters finally exceeded the conversion capacities, which were naturally present. The ecosystems lost their balance, which resulted in smelling surface waters and disappearing fish populations.
In the 1970s, with the imposition of high fees for discharging industrial wastewater, biological wastewater treatment for industrial effluents became an important topic in Europe. At present wastewater treatment systems are often an integral part of production facilities.
In biological wastewater treatment systems, the processes related to the conversion of organic pollutants, which normally take place in receiving ecosystems, are now performed in tanks.
However, the conversion rates in these tanks need to be much higher as compared to natural systems. This can be achieved through the immobilization of bacteria. Process conditions in these reactors can be chosen in such a manner that bacteria tend to attach to each other, producing aggregates, flocs, or granules. In contrast to free-swimming bacteria, these aggregates have favorable settling, flotation, and attachment properties.
These properties now are the key to successful biological wastewater treatment. By means of specific settlers, carriers, or flotation processes, the biomass is retained in the reactor. In this manner, high biomass concentrations are maintained allowing high volumetric conversion capacities.
Anaerobic and Aerobic Wastewater Treatment
During the same period as the introduction of discharge fees for industry, the Upflow Anaerobic Sludge Blanket (UASB) system was developed at Dutch Universities. Due to the good settling properties of the granular sludge, it was possible to design a compact system with high biomass concentrations in which organic pollutants could be converted into biogas. This allowed industrial effluents to be treated cost-effectively. Until that time the wastewater treatment technology most applied concerned aerobic Activated Sludge (AS) treatment. These systems had relatively low volumetric conversion capacities and very high energy requirements due to aeration. An added bonus with anaerobic technology is that only very small amounts of biological sludge are produced.
With anaerobic technology 70-90% of the organic pollutants are removed. If however complete purification (up to 99 %) is aimed for, aerobic post-treatment is required, sometimes in combination with effluent polishing steps such as sand filtration or membrane technology. In that case, also nutrients like nitrogen may be removed and discharge to surface water becomes an option. At present, most industrial wastewater treatment systems concern combinations of anaerobic and aerobic technology, allowing cost-effective treatment whilst producing high-quality effluents.
Biomass Retention
As already mentioned, effective wastewater treatment relies on the phenomenon of aggregation of the bacteria present in a reactor system.
Aggregates can be retained in the reactor system by means of settlers, flotation processes, or carrier-based technology. These systems are either an integral part of the reactor system itself, like the internals of UASB and EGSB reactors, or represented by the carriers of the specially designed and patented DACS, DANA©, and AGAR©-MBBR systems. The prevailing concept in conventional (aerobic) activated sludge systems is settlers which are constructed as separate process units.
The only systems where we can find cultures present as suspended freely swimming bacteria is in Continuous Stirred Tank Reactors (CSTR), both in the fermentation industry and in microbiological laboratories.
In nature aggregation or biofilm formation of microorganisms seems to be the rule rather than the exception and is often preceded by attachment to organic or inorganic surfaces or colloidal particles. Some form of aggregation is a necessity for survival in streams, rivers and soils as the microorganisms would otherwise be “washed out of the system”. This illustrates one of the most important selective process parameters inducing aggregation, dilution rate, or washout rate in a reactor system.
Different types of aggregates are encountered in wastewater treatment systems. Both granules and flocs are found. This is important as the type and density of the biomass will determine the retainability of biomass and finally determines the biomass concentrations feasible in a reactor. At relatively low flow velocities flocculant biomass is still retained in a reactor while at higher flow velocities there is an increased selection pressure for aggregating bacteria (granule formation). Under these latter circumstances, it is also observed that the bacteria react by attaching themselves to the reactor wall or carriers. Apparently, the only manner in which the bacteria can survive in the system is through aggregation and production of biofilms, which are not susceptible to washing out.
Granules are encountered in anaerobic granular systems like UASB and EGSB reactors and in aerobic granular systems like the Nereda SBR. The DACS system and aerobic MBBR and Activated Sludge systems typically contain flocculant biomass; where for DACS and MBBR this biomass is attached to the specifically designed carrier.
Operational Window
Granule formation is not simply induced by a certain flow velocity, it also requires ideal wastewaters characteristics defined typically by the “operational window” of a system. Especially pH, suspended solids content, conductivity, and salt concentration need to be within a certain range. Additionally, granular sludge-based systems often have an issue with too high COD (Chemical Oxygen Demand) concentrations limiting the formation of granules. If any or several of the above parameters is out of the operational window, granules may not form or start to disintegrate, and flocculant biomass is formed which may be lost due to washout by the (too) high flow velocities in the reactor. This phenomenon is more pronounced in EGSB reactors which are typically 15-24 meters high, working with higher upflow velocities.
The operational window of granular sludge-based systems will be more and more limiting as the upflow velocity in the reactors increases. UASB systems for example typically will use upflow velocities of 0.8-1 m/h while so-called high rate anaerobic EGSB systems may work with upflow velocities of 4-6m/h. The sizes of granules vary between 0.5-3 mm and are retained in the reactors by controlling the upflow velocity. When for whichever reason the effluent characteristics change and cause the granules to disintegrate these systems’ capacity is significantly reduced, or they completely cease working due to wash-out of valuable biomass.
Although the average effluent of most food and beverage (including beer) industries will fit the operational window of granular sludge-based systems, many times peak flows, accidental discharges, changes in production lines, and respective CIPs will cause granules to disintegrate. The fact that disintegration is a recurring problem in granular sludge-based systems is substantiated by the growing trade in granular sludge to solve emergency situations.
Anaerobic carrier-based systems have been developed to overcome the necessity for granular sludge formation and thus warrant the continuity of anaerobic treatment systems within a significantly larger operational window. Aerobic carrier-based systems have the advantage of being able to combine several biological processes in a reduced volume, as compared to conventional AS systems, without the necessity for biomass recirculation.
Both anaerobic and aerobic carrier-based systems and their technological, operational and commercial advantages, compared to more conventional technologies, will be discussed.
Wastewater treatment in the beer and beverage industry
The quality and quantity of brewery effluent can fluctuate significantly as it depends on various processes that take place within the brewery (raw material handling, wort preparation, fermentation, filtration, CIP, packaging etc.). The amount of wastewater produced is related to the specific water consumption (expressed as hl water / hl beer brewed).
In general, smaller breweries will only have a simple pre-treatment system for their effluent consisting of solids screening and a mixing and compensation tank after which it is discharged into the municipal sewer. Some mid-sized companies (>150 000-200 000 hl) do have aerobic systems like e.g. SBR (Sequencing Batch Reactor), but also still no anaerobic system. Because organic matter concentration in brewery effluent is significant a high input of energy for aeration is required. Another cost factor is the amount of waste sludge generated from aerobic metabolism, which also needs to be handled and disposed of. Both increase the cost of operation of the treatment system. Therefore, anaerobic treatment processes are being implemented more and more frequently because energy is saved, own renewable energy can be earned, and sludge disposal costs are minimized. However – until today – this technology has only made sense for the larger breweries with a Break-Even Point (BEP) of about 1500 kg COD/d and a comparatively long Return on Investment (ROI) of about 8-10 years.
Downflow Anaerobic Carrier System – DACS
Knowing the history of conventional anaerobic wastewater treatment solutions (based on granular sludge) with their respective challenges, it is obvious that the industrial wastewater treatment sector has always been looking for ways to improve this segment. In 2008-2009 Aqwise (market leader in MBBR solutions) and Dutch Water Technologies Group (DWT) started their first experiments with an anaerobic carrier-based technology. They found that the patented Aqwise carriers which have a specific open structure were extremely suitable for anaerobic biomass to attach to, and provided a “house” for the flocculant aerobic bacteria to grow in. In addition, it was found that the system worked best, producing biogas with very high methane content, in a DownFlow reactor (fig. 1). After having conducted several on-site tests for different industries they decided to create a joint venture in the Netherlands called Aqana, which was to further develop the technology and start implementing it in different market sectors. The name they gave this new technology was, logically, “DACS”.
DACS Working Principle
The DACS reactor is configured for downward water flows, from the top to the bottom (downflow) of the reactor, passing through a bed of floating plastic carriers (fig. 2). The anaerobic biomass is attached mainly to the inside of the carrier. As with conventional anaerobic technology the organic matter in the wastewater is converted by the anaerobic biomass into biogas and a relatively small amount of anaerobic sludge. In contrast to the wastewater, the biogas rises upwards to the top of the reactor, providing a gentle mixing of the carriers and good distribution of water throughout the carrier bed (fig. 3). By using the carriers, 3 phase separation (biogas/water/solids) takes place without the addition of complex internals or additional process units. Biomass required for treatment is maintained in the reactor by the carriers and excess sludge is removed.
Carrier Material
In contrast to traditional anaerobic reactors the DACS process supports the anaerobic flocculant biomass by using carriers submerged in the water phase (fig. 4). After inoculation with seed sludge (biomass) the biomass population forms within the carrier. The specific density of the carriers – slightly lower than water – ensures flotation of the carrier biomass bed inside the reactor. This also enables the concept of the downflow configuration. The fluidized property of the carrier bed enables ideal mass transfer at high down-flow velocities and ensures biogas release in the upwards direction.
High methane content of the biogas
In the DACS the wastewater flows in opposite direction to the rising biogas. The incoming wastewater is equally distributed over the carrier’s bed by the use of sparge bars installed in the gas phase. The contact of water with biogas allows absorption of CO2 in the water fraction. Combined with an increased water pressure of downflowing wastewater, the ability of CO2 absorption increases. By the absorption of CO2 in the water, a higher concentration of methane is present in the biogas. DACS-produced biogas typically has a methane content of 80-90%.
Added Values for the Industry
Since 2011 DACS technology has been applied not only in brewery, food and beverage, and pulp and paper industries but also in the chemical industry. The results showed significant advantages and improvements, as compared to conventional technologies, in various aspects:
■ As a result of the absence of complex internals, the possibility is opened to different configurations of the DACS reactor tank. Therefore, retrofitting existing process tanks and old UASB or EGSB reactors is usually possible within a certain range of applicability.
■ DACS can be applied where other anaerobic technologies have issues with the disintegration of granular sludge due to e.g. high COD and/or salt concentrations; for example at distilleries and chemical industries;
■ DACS is able to handle similar loading rates as compared to high-rate EGSB reactors (Volumetric Loading Rates (VLR) vary from 20-25 kg COD m3/d);
■ temporary shock loads, caused e.g. by accidental discharges of chemicals or high COD loads, did not cause any significant disturbance of the biomass; biomass may be somewhat affected for a short period of time but was always able to recover within a few days; no reseeding was required;
■ since biomass is “housed“ in a floating carrier, higher suspended solids loads or accidental discharges with higher solids loads do not affect the biomass; solids are simply washed out of the system and do not “compete“ for volume causing washout of valuable biomass;
■ the biogas produced through DACS technology typically contains 80-90% methane, which means factory boilers can run more efficiently since less CO2 is present in the biogas;
Last but not least it is important to mention that up till now no DACS installation has required re-seeding with biomass because of sludge loss. All DACS systems which had to run for a longer or shorter period in adverse conditions were all able to fully recover within a short period of time.
AGAR® (Attached Growth Airlift Reactor) MBBR
The AGAR is a fixed biofilm moving bed process that uses suspended biomass carriers with extended surface area for biofilm growth, along with carefully designed reactor hydraulics. The aerobic AGAR process can fit in most activated sludge systems and can be implemented in a variety of configurations. The Attached Growth Airlift Reactor technology combines a unique fully open and fully protected biomass carrier with highly efficient aeration and mixing design. This results in superior effective surface area for biomass growth and optimal oxygen transfer efficiency.
The AGAR IFAS (Integrated Fixed film/ Activated Sludge) configuration combines biofilm growth on biomass carriers with suspended growth in the activated sludge (fig. 5, 6). The AGAR IFAS process can be used for upgrading existing plants for biological nutrient removal and increasing treatment capacity, without adding reactor volume.
An AGAR IFAS reactor is divided into aerobic, anoxic, and anaerobic volumes, similar to a conventional activated sludge system. The AGAR IFAS biomass carriers are retained in the aerobic stages by screens located at the effluent end of the process stage. Competition between suspended biomass and fixed biomass leads to the execution of completely different functions within the same volume: suspended biomass is mainly assimilating BOD/COD, whereas fixed biomass is mainly performing nitrification. Nutrient removal can be precisely controlled by determining the size, location, and quantity of biomass carriers in each of the process stages using advanced modeling and in-depth process design tools. The quantity of carriers required to achieve the required effluent quality in each zone is calculated, and the final sizing of volumes and quantities of carriers are optimized. The required oxygen and resultant aeration requirements are calculated, and the sizing and layout of the aeration diffusers is planned.
Carriers
The also patented Aqwise Biomass Carriers (ABC5, see fig. 7) have several important advantages compared to other carriers available on the market. These carriers have the largest protected surface area (per unit volume) of all equivalent carriers, and as a result, require the smallest volume of required carriers to achieve a given effluent quality. The shape has been optimized for perfect mixing. Therefore, the reactor filling ratio can be up to 70 % of the reactor volume, if required. This means the same tank volume will be able to treat growing pollution loads, or conversely, produce higher quality effluent, through the addition of carriers alone. The shape is made such that the carriers do not clog, do not stick one to another, and move very easily in the water.
Aqwise’s biomass carrier is unique in its geometrical fully open shape compared to other carriers in the market. This “fully opened” shape allows the oxygen and substrates (BOD/COD and/or e.g. NH4) to easily come into contact with the entire biomass and complete the full aerobic reactions while avoiding anoxic or anaerobic conditions. Furthermore, due to these openings, excess biofilm is sloughed off from the carrier so that the carrier doesn’t get clogged.
AGAR Aeration
The unique AGAR aeration design is using either fine or coarse bubble diffusers to create a double role motion in the reactor. The double role motion creates constant collisions between the carriers and keeps a thin and viable biofilm on the carriers. Due to the unique aeration design – double roll pattern, in most cases, no additional air is required for the rolling motion and air supplied to fulfill the biomass demands is sufficient for both biological activity and mixing.
Retention of Carriers
Screens are installed at the outlet of the biological stage in order to maintain the carriers inside the reactor, whereas mixed liquor may flow unobstructed (fig. 8). These wedge wire screens are continuously cleaned by the mixing flow of water, air bubbles, and biomass carriers.
Added Values for the Industry
■ Smaller footprint compared to similar technologies through patented “fully open” carrier design allowing full use of the surface area;
■ integration of carriers into a given reactor volume containing activated sludge (mixed liquor) increases the absolute amount of biomass in the reactor, simply by adding the biomass growing on the carriers to the suspended biomass floating on the mixed liquor;
■ mixing by “double roll” patterns: the mixing system of consecutive “double roll” patterns developed by Aqwise allows an optimized mixing of the carriers for a minimum quantity of air, resulting in lower energy consumption;
■ robustness – fast recovery of the biological process in case of toxic and hydraulic shocks; by “housing” the bacteria in a carrier this special feature applies to both anaerobic and aerobic technologies using carriers;
■ carriers are available as “virgin” material for food grade applications or recycled material, with or without UV protection. Carriers are manufactured in the EU with the highest standards.
Reduced footprint through the dynamic anaerobic-aerobic (DANA) wastewater treatment plant
The Dynamic Anaerobic Aerobic treatment or DANA was developed by Aqana to meet the requirements of clients with limited space available for the implementation of their wastewater treatment facilities.
The DANA combines DACS and AGAR technologies in the most compact way. In figure 9 it can be seen that the aerobic AGAR part of the plant is installed on top of the anaerobic DACS reactor. The DANA combined anaerobic-aerobic treatment plant has one of the smallest footprints currently available on the market (fig. 10).
Conclusion
Breweries are one of the traditional industry branches in the agro-industrial, food and beverage sectors using cost-effective and feasible techniques also in wastewater treatment plants to manufacture the best quality product in a more and more sustainable manner. Wastewater has meanwhile become a valuable substance and the historical development of various technologies conducted to various high-performance systems as laid down in this article. Major biological and technical influencing factors as well as impact coherences of an efficient and satisfying operation are described. The combination of anaerobic and aerobic technologies was studied. The two-stage technology is more effective for the removal of organic pollution and suspended solids, while under optimal conditions even nutrient removal can be achieved. Anaerobic treatment is a meanwhile widely applied method for the treatment of brewery effluent. Combined anaerobic/ aerobic treatment of brewery effluent has important advantages over complete aerobic treatment especially regarding a positive energy balance, reduced (bio) sludge production, and significant low space requirements. Brewery industries are mainly small and medium, less common large enterprises but with a significant social and economic value. Therefore, their sustainability policy (including the growing use of renewable energy sources/potentials like wastewater) requires wastewater treatment systems with the best performance and the fact is that well-known processes and technologies are available for such purpose. In order to meet strict constraints with respect to space, odors, and minimal sludge production, considerable attention has been directed towards the anaerobic/aerobic reactors.
Facing some still existing optimization potentials of the existing conventional technologies in this article a new generation of patented aerobic and anaerobic WWTP systems is illustrated and its added values for the beer and beverage industries are positioned. Meanwhile, more than 500 references in 50 nations have globally proven to perform in adverse conditions where conventional systems reach their limits. Lower values in BEP, TCO, ROI, CAPEX, and OPEX besides more robust operation with higher KPIs in energy earning and operational continuity and stability are the main benefits and established results.
