Biological Odour Control at Wastewater Treatment Facilities - the present and the future (let's forget the past)

N.J.R. (Bart) Kraakman1

1Technical University Delft, The Netherlands 1Bioway Technologies Australia Pty Ltd, Australia

(email: n.j.r.kraakman@tudelft.nl or b.kraakman@bioway.com)

ABSTRACT

Biological treatment to control odours at wastewater treatment facilities went through a mayor development step in the last 15 years. The development of more advanced biological systems to control odours at wastewater treatment facilities solved many design and operational stability limitations with conventional biofilter type systems. With the use of more reliable cost effective biological treatment methods combined with the use of more objective tools to quantify odours, odour emissions from wastewater facilities can now be controlled more simply and predictably. This paper discusses the developments, the advantages and limitations of biological odour treatment technology at wastewater collection and treatment facilities.

INTRODUCTION

Over recent years, much progress has been made in areas related to biological gas treatment such as microbiology, process modelling, reactor design and reactor-operation. As a result, more advanced biological gas treatment systems have been developed to solve problems of odorous or polluted air emissions for different industries of which many have been applied successfully in full scale. New bio-engineered systems have made it possible to extend the application field of biological waste gas treatment technology. Experiences with biological odour control at wastewater collection and wastewater treatment facilities ranging from hot climate condition like Australia and the Middle East to colder climate conditions like North Europe and Canada have led to improvements in especially efficiency and reliability. Besides better process control, higher degradation capacities also lower operational costs and smaller footprints are some of the advantages of the newer biological technologies for odour control in the water industry.

This paper will outline the developments of biotechnology for the control of odour emissions at wastewater treatment facilities. The advantages and limitations of biological treatment to control odours at wastewater facilities are shown. Examples of full-scale operating experiences are given and issues related to design,

implementation and operation are discussed. DEVELOPMENTS OVER THE YEARS

Introduction Biological treatment to control odours at wastewater treatment facilities went recently through a mayor development step. In the early days, the design and operation of the most applied system, a biofilter, was done mainly by try and error. Over recent years, much progress has been made in many areas such as microbiology, process modelling, reactor design and reactor-operation (Kennes and Veiga 2001, Shareefdeen and Singh, 2005). The conventional biofilter has changed from being a black box into better defined biological systems with better control of the biological treatment process.

Diversification of the technology Various reactor configurations have been developed for different applications. The biological techniques for wastegas treatment are traditionally classified as biofilters, biotrickling filters and bioscrubbers. In the wastewater industry, biotrickling filter type systems are now-a-days the most common, but are often also referred to as bioscrubbers. But besides biotrickling filter and biofilters also injection of odorous air into the aeration tank of a wastewater treatment plant is used for treatment of low airflows with typically high strength odours.

Conventional biofilters use many types of organic material as support for the micro-organisms and sometime the biofilter media is mixed with granular, inorganic materials (e.g., lava rocks, clay balls or perlite) to stabilize the structure and to prevent preferential air flows and to increase the life of the media. The conventional biofilters using organic or partly organic media are applied less frequently as they face important design limitations and operating stability problems. The odorous air from wastewater treatment systems usually contains hydrogen sulphide, which is oxidised to sulphuric acid in a biofilter system. Sulphuric acid accumulates in the conventional media types as it is difficult to wash with water, reducing the overall odour removal efficiencies over time. Also, the odorous air streams are nearly always not completely saturated with water, which leads to partial drying out of the media,

especially in the inlet, (usually bottom), part of the biofilter. Moisture conditions are very critical in a biofilter and a drop in moisture as result of uneven irrigation or inaccurate humidification will result in a reduction in the so called water activity of the media reducing the overall efficiency. Too much irrigation can also result in premature media decomposition, anaerobic zones, preferential airflow distribution and an increased pressure drop.

The design and operational stability limitations of conventional biofilters including the requirement of media replacement are solved with the development of more advanced biological air treatment systems. In the more advanced biological odour treatment reactors the media change-out or cleaning is less or no longer required, since inert preferable structured material as support media for the micro-organisms is used. Bu also the control of important conditions for the biological process is improved. Although the more advanced systems require usually higher investment cost, the operational costs are greatly reduced, since up to 40% of the operational cost of a conventional biological system is typically related to the media change-out. Also pressure drop is usual lower but especially more stable over time resulting in reduced energy consumption for the fans.

The probably most important development is that the more advanced biological system are more operator friendly as the operation is more predictable and the controls are simplified. Equipment should preferably be simple and reliable to operate. Robustness is essential for any technology when applied, but is especially important for biotechnology. The microbiological community in a biological treatment system will face fluctuations related to the process upstream as a result of continuous or discontinuous production, irregularly unplanned shut downs, planned maintenance shut-downs and diurnal fluctuations. There may also be fluctuations related to the operation of the system, for example, associated with loss of control of power, water or nutrient supply. Quantification of the robustness of a biological air purification system has helped designers and operators. Robustness can be defined to reflect the ability of the biological system to deal with fluctuations and operational upsets and examples are given elsewhere (Kraakman, 2004 and 2005). Microbial responses to stress conditions are important to quantify. Biological air treatment systems using mixed microbial cultures as inoculum are self-optimising with species becoming dominant that are most competitive under the environmental conditions in the system. Unfortunately this self-optimising adaptation process seems to be relatively slow and is likely to take months or longer. On the other hand, many full-scale

applications and labscale tests showed that a biological system can deal with spikes. Experiences show that a biological system can handle very well the diurnal occurring peaks typical for odour emissions from wastewater processes. When temporary reduced removal efficiencies of the odour control system are noticed, its often the first day after many days of wet weather when the relatively low concentrations reduce the (enzyme) activity of the microorganisms over a couple of days. Recovery to full activity is fast and is normally a matter of minutes to hours rather than hours to days. Quantifying the robustness and understanding the risks has helped designer to implement measures like duty/standby equipment items and back-up water supply systems with nutrient dosing if necessary to reduce the risks and obtain an odour treatment system that is predictable in operational costs and friendly in operation.

Biological wastegas treatment is nowadays especially applied to control odours at wastewater treatment facilities or composting and rendering facilities, but has been applied more and more at other industries like the food industry, pulp and paper industry, the chemical industry.

Figure 1: An example of a conventional open bed biofilter during smoke testing visualising air distribution quality.

Figure 2: An example of an enclosed conventional biofilter showing the low profile to minimize the overall pressure drop (energy consumption).

IMPROVEMENTS IN DESIGN AND OPERATION

Introduction Biological gas treatment systems convert pollutants from the air into water, carbon dioxide and salts. Micro-organisms, primarily bacteria, are the catalyst of this process. The inlet odorous airstream provides an ongoing source for oxygen and energy (the odorous compounds) for the biological process. The overall process in a biological gas treatment system can be usually divided in two phases: the mass transfer of the pollutants from the gas phase to the micro-organisms, and the biological degradation of the pollutants. The combination of different physical, chemical and biological mechanisms results in a relatively complex system. Fundamental parameters including mass transfer, absorption of the different pollutants and degradation kinetics in the biofilm, as well as airflow and water distribution are often difficult to quantify. Much progress has been made in understanding the fundamental aspects, which are necessary for design, implementation and operations. Still much of the current design work, on both sizing and operations, is based on empirical experience. Models have been greatly improved but are still not always applicable, especially for the treatment of compound mixtures and varying concentrations. Table 1 shows a summary of important improvements that have been made on biological odour control technology over the last 15 years. All these improvements resulted in systems which are now more reliable and simpler to operate and require less costs to operate. Below are discussed some individual aspects of biological treatment technology that are critical for the treatment of odorous gases at waste water collection systems and waste water treatment plants.

Moisture control The lack of a mobile water phase in a conventional biofilter makes the control of conditions important for microbial activity, such as water activity, pH, salt content and nutrient concentration more difficult. The more advanced biological odour control systems do have a mobile water phase, which is used to directly measure and control the biological process. The most important process parameter for biological process in a biological odour treatment system at a wastewater treatment facility is the moisture content. An optima wet environment for the bacteria is essential and often underestimated during design and operations. Moisture conditions are very critical in a biofilter and a drop in moisture as result of uneven irrigation or inaccurate (pre-)humidification will directly result in a reduction in the so called water activity of the media reducing the overall efficiency within minutes. At least 50%, but probably close to 75%

of the problems with conventional biofilters are related to a poor control of the water content in the biofiler media.

Nutrients Other important process conditions include pH, nutrients, saltcontent and temperature, which the optimum value differ for the many microorganisms that are required for complete treatment of the gas. Usually secondary effluent water from a wastewater treatment plant is used, because it contains all the essential nutrients and trace elements necessary for the biological conversions. Secondary effluent water from wastewater treatment can be used most of the times as long as the COD content and total suspended solids are not too high (approximately less than respectively 100mg/l and 30 mg/l). With more stringent water effluent requirements from a wastewater treatment facility, more advanced wastewater treatment steps are used (e.g. Membrane BioReactors). It that case, the level of especially nitrogen and phoshate can become too low and additional nutrient dosing might be required.

Temperature Experiences with biological odour control at wastewater facilities ranging from hot climate condition like Australia and the Middle East to colder climate conditions like North Europe and Canada have all led to increases in both efficiency and reliability in relation to temperature. Temperature is an important process parameter for the biological conversion, which a general rule of dumb that between 5 and 45 degrees Celsius for every 10 degrees the biological activity is increased by a factor two. The overall increase in efficiency in a biofilter system is not always increased by a factor two as overall process in a biological gas treatment system can be divided in two phases: the mass transfer of the pollutants from the foul air to the micro-organisms, and the biological degradation of the pollutants. Biology activity is in general increased by an increase in temperature, but the mass-transfer can be reduced by an increase in temperature as the Henry-coefficient changes. Some research (not published) has shown that at low concentrations and mass-transfer limiting conditions a decrease in temperature actually increased the overall removal efficiency of the biological gas treatment. Temperature is an important factor for the design (size) of a system, but less of important for the operations of a biological odour treatment system. But at extreme temperature like in the Middle East, a biological system is more sensitive to sub-optimal moisture control as evaporation rate are much higher.

Multiple compounds Odorous air streams from wastewater processes

contain a mixture of many compounds. These various compounds usually have very different chemical properties as can be seen from, for example, their water solubility and their biodegradability. To degrade all odorous compounds an optimal mix of microorganisms is required. Micro-organisms differ from each other in their capacities to obtain their energy, carbon and nutrients. Micro-organisms also differ in their ability to form a good biofilm structure, their growth-rate, their affinity for compounds, their degradation capacity, and their nutrient requirements. Unfortunately the optimal environmental conditions for the micro-organisms also differ. Therefore due to the many different compounds in the air stream, a mix of micro-organisms is required and different environmental conditions are preferred in the biological odour treatment system.

When an air stream contains multiple compounds, it can be expected that the removal of many of the compounds is affected by the presence of other compounds. In the situation that a bioreactor system is removing two or more compounds, the metabolic activity in a micro-organism may involve the mechanism of induction, inhibition and sometimes co-metabolism. Induction refers here to the process that initiates the production of enzymes that catalyse the biodegradation in the cell. Inhibition involves the toxicity effect of certain compounds on the metabolic activity of the micro-organisms and co-metabolism is the (partial) conversion of certain compounds by enzymes that are induced by other compounds. The mechanism for micro-organisms to ensure that the organism uses the more readily catabolisable carbon and energy source is called catabolite repression. One consequence of catabolite repression can be that if more compounds are present at the same time, the metabolism of a certain compound is resumed only after another compound causing catabolite repression is removed first (socalled diauxic growth). Therefore it is often a necessity in a biological system that certain compounds are removed first, before other compounds can be removed.

Biological odour treatment reactor at wastewater collection and wastewater treatment plants often deal with many odorous compounds, among them mixtures of volatile reduced sulphur compounds, like H2S and mercaptans, which are important because of their very low odour threshold. Although different micro-organisms are known for the degradation of volatile reduced sulphur compounds, the treatment of an air stream containing mixtures of reduced sulphur compounds remains challenging for two main reasons. Firstly, the energy yielding process of H2S oxidation is higher and thus preferred over the oxidation of other reduced sulphur compounds. Secondly, the degradation of many of

these sulphur compounds is only possible with high efficiencies at neutral pH, while a degradation product from sulphur compounds is sulphuric acid which reduces the pH. Different types of organisms require different environmental conditions including the absence of easily degradable compounds and are, therefore, preferably separated in different layers of the reactor. Multi-stage biological odour treatment systems have therefore important advantages over single stage systems.

Instead of applying multi-stage biological treatment, polishing with, for example, activated carbon is now often applied to obtain low outlet odour concentrations. This is normally not necessary when the biological odour treatment system is designed well and multi-stage systems are used. The elimination of activated carbon polishing reduces complexity and the risk of higher and unpredictable operating costs involved with the change-out and the disposal of the activated carbon and thereby improving overall sustainability of the odour control system.

Reactor and media configuration The reactor size and shape and the configuration of the internal carrier (media) for the micro-organisms directly influences the removal capacity of the biological system and will impact important design parameters such as mass-transfer rate, bacterial degradation capacity, water holding capacity and pressure loss. Often underestimated is the influence of the reactor and media configuration on the air flow characteristics through the biological gas treatment system. When high removal efficiencies are required (>99%), all the air has to be treated effectively. In order to treat the air effectively, all the air needs to be in contact with the micro-organisms for a minimum period of time to exchange the compounds to the micro-organisms. An optimum air distribution in the reactor is required and preferably moves the air through the reactor as a plug-flow. Preferential air streams or partial by-passes of the air should be avoided at all times. In present theoretical models describing the degradation of gas components in biological filters, air flow is described as a plug-flow, but this theory is oversimplified (Prenafeta-Boldu et at., 2008). The air flow behaviour has considerable effect on the maximum achievable outlet concentrations and thus overall cleaning efficiency. A reactor with one layer of randomly packed media is likely to be subject to sub-optimal air distribution through the media especially after a period of operating time. Instead of randomly packed media, structured media can prevent this as structured media makes the operation more determined by design rather than chance. The media used should be preferably inert and not subject to blocking, fouling, erosion or corrosion causing, for example, shrinking of the media or

preferential airflows through the media. The method by which water is added to the reactor is also very important as water not only prevents the biofilm layer in the biological system from drying out, but also serves as a supplier of nutrients for the micro-organisms in addition to removing the degradation product, often sulphuric acid. Water can be recirculated over the media or can be added to the top of the media and then, after passing once-through the media, be directly removed from the bottom of the system. Minimising water use in the reactor can have benefits. First, a thinner water film can be maintained on the biofilm layer, which is often preferred to minimise the resistance for mass transfer of the pollutants from the gas phase to the micro-organisms. Secondly the thinner water film on the media results in a larger void volume in the media. A larger void volume is better as the pollutant airstream will be in contact with the biofilm longer, resulting in a longer so-called actual residence time. The actual gas residence time is a better design parameter than the often used theoretical empty bed gas residence time (Theoretical EBRT) as it represents the real time in seconds that the air is in contact with the biology. Theoretical EBRT = media volume (m3) / air flow (m3 sec-1) (sec) Actual RT = media volume (m3) x void fraction (%) / airflow (m3 sec-1) (sec)

Start-up The disadvantage of using synthetic media is that the start-up time might be longer than the conventional biofilters using organic media types, which already contains micro-organisms. Start-up times can vary from a couple of weeks to a couple of months to obtain optimal and stable performance. Addition of activated sludge and/or pre-grown micro-organisms is frequently applied to overcome long start-up times.

Figure 3: An example of a multi-stage biological odour treatment system requiring less operating costs as a result of reduced pressure drop and the absence of media replacement.

CHALLENGES FOR FUTURE DEVELOPMENT

Biological gas treatment technology has been demonstrated and accepted by regulators and managers in the water industry as cost-effective and reliable, when designed and operated properly. Especially biotrickling and bioscrubber reactors have extended this field of biological gas treatment over the last years. Further improvements of biological air treatment technology can be explored by improving the current state-of-the-art technology. Development should be focussed on issues like: poor water-soluble compounds; larger pollutant loadings; further improve the robustness (predictability to process upsets).

To increase performance, the rate-limiting step could be defined better for different waste gas streams; normally the biological degradation rate or the mass transfer rate. When mass transfer is limiting, simpler tools still need to be developed to determine what exactly is limiting (oxygen, pollutant or accumulated degradation intermediates or products) and where exactly limitation occurs (waterfilm, biofilm and inhomogeneous air distribution).

Flow characterization in a biotrickling or bioscrubber reactor is important since gas flow, liquid flow and gas velocity have an important impact on process parameters like mean gas residence time, gas dispersion in the reactor and pressure drop over the system. These parameters are important to scale up and to operate a reactor at optimum are will be to be developed to a greater extent.

Models, although improved, still fail to predict short time operation robustness and long-term performance. Further research is required to transform the technology of biofiltration from an empirical practice to a more theoretical base technology.

CONCLUSION

The wastewater industry prefers odour control methods that are relatively maintenance free, easy to operate and sustainable (low energy input, chemical free, predictable low operational costs). Biological treatment to control odours at wastewater treatment facilities went through a mayor development step in the last 15 years. The development of more advanced biological systems to control odours at wastewater treatment facilities solved many design and operational stability limitations with conventional biofilter type systems. In the more advanced biological odour treatment systems the media change-out or cleaning is no longer required and the reliability greatly improved. With the use of

more reliable cost effective biological treatment methods combined with the use of more objective tools to quantify odours, odour emissions from wastewater facilities can now be controlled more simply and predictably.

REFERENCES

Kennes, C. and Veiga., M.C. 2001. Bioreactors for waste gas treatment. Kluwer Acadamic Publicers, dordrecht, The Netherlands.

Kraakman, N.J.R.. 2004 H2S and Odor Control at Wastewater Collection Systems: An On-site Study on the Robustness of a Biological Treatment. Poster presented at the 2004 USC-CSC-TRG Conference on Biofiltration, Santa Monica, USA, October 20-22, 2004.

Kraakman, N.J.R. 2005. Biotrickling and bioscrubbers applications to control odor and air pollutants: developments, implementation issues and case studies. Biotechnology for Odour and Air Pollution Control. Springer-Verlag, Heidelberg, Germany. Edited by Shareefdeen, Z. and Singh, A. :355-379.

Prenafeta-Boldu, F., Illa, J., van Groenestijn, J.W., Flotats, X. 2008. Influece of synthetic packing materials on the gas dispersion and biodegradation kinetics in fungal air biofilters. Appl. Microbial Biotechnol, 7: 319-327.

Shareefdeen, Z. and Singh, A. 2005 Biotechnology for odour and air pollution control, Springer-Verlag, Heidelberg, Germany.

Table 1: Improvements of biological odour control system at wastewater facilities over the last 15 years. Improvements Benefits

the introduction of a mobile water phase A better control of important microbial parameters.

the use of synthetic support A larger void volume (smaller reactor systems), more stable long term operation (less or no media change-out), lower pressure drop (less energy required).

improved water and air distribution A smaller reactor systems and lower outlet concentrations.

phase separation of gas transport and biological activity

A lower pressure drop (lower energy required) and/or taller reactor systems (smaller footprint with improved dispersion).

multi-layer approach A better use of biological capacities and lower outlet odour concentrations.