• About this site|
• Site index|
• Contact us|
• Jobs

Good Practice Guide for Assessing and Managing Odour in New Zealand

Foreword

Acknowledgements

1 Introduction

2 Properties and Effects of Odour

3 Legislation and Case Law

4 Odour Assessment

5 Odour Management and Control

Appendix 1: Odour Assessment Decision Trees

Appendix 2: Sample Survey Questionnaire

Appendix 3: Odour Management Plans

Appendix 4: Example Odour Diary Record Sheet

Appendix 5: Odour Control Systems

Appendix 6: Odour Thresholds for Individual Chemicals

References

About the Ministry

5 Odour Management and Control

5.1 Management options for regulators

5.1.1 Regulation

Regulators manage odour emissions through policies and rules in regional and district plans. The rules typically specify those discharges that are permitted (usually subject to certain conditions) and those that require a resource consent. Monitoring and responses to odour complaints also play a significant role in managing potential odour impacts of existing activities. The options available to regulators are largely provided under the RMA, as discussed in section 3.1. Resource consents may be granted subject to conditions, as discussed in section 3.4.

District councils can manage odour effects through controls on land use. Separation distances can be considered in line with the principles of internalisation and reverse sensitivity discussed in section 3.3. Buffer zones can work both ways; for example, protecting existing residential activities from a new pig farm moving in, as well as stopping residential subdivision and development from occurring around an existing pig farm.

Further guidance on establishing buffer zones for different activities can be found in VEPA (1990) and ARC (2002).

Guidance on writing air provisions in plans is provided on the quality planning website at: www.qualityplanning.org.nz.

5.1.2 Monitoring and enforcement

Local authorities are required to monitor, respond to and keep records of complaints to effectively carry out their functions. Guidance on methods for monitoring to assess compliance and for responding to complaints is provided in section 4.

For existing sites, where complaints have been received a decision must be made about whether there is an adverse effect occurring from offensive and objectionable odour. In these cases, assessments recommended for category 1 activities in section 4.2.1 should be undertaken to determine any adverse effect of odour (see Appendix 1 for the relevant decision tree).

It can be difficult to decide when to take action for non-compliance with an odour effect condition. Complaint response can be a drawn-out process, particularly where there is uncertainty and complaints cannot be validated. Other assessment tools may be needed to gather more information.

Community assessment techniques (complaints, diaries, surveys and meetings, as described in section 4) are the highest priority for assessing existing odour sources. These should be used as the primary information source in decisions about what action to take. Tools typically used to determine potential adverse effects such as modelling and the existence of management procedures should not be a consideration, although modelling can be used to interpret complaint and survey data.

Odour complaint response generally tells an officer on a particular occasion whether he or she observed an odour that could be considered to have an impact (such as the ratings in Table 4.4). Diaries, surveys and complaint history can give an indication of the cumulative impact over time (chronic effects).

The ultimate action a council takes will depend on the policy of the individual council, the history of the site, the degree of adverse effect and how much co-operation there is from the discharger.

Enforcement action is usually the last resort and it is most often supported by complaint data. Such cases can be lengthy and leave the community dissatisfied, particularly for chronic effects. Some councils have been quick to take enforcement action on the basis of relatively few complaints. This is generally easier for acute effects or where the discharger has clearly been negligent. In other cases, it is more appropriate to provide a warning and allow the discharger time to rectify problems.

Figure 5.1 summarises the information and actions that should be considered when an adverse effect from odour has been confirmed.

Figure 5.1: Information and actions considered for odour response

Methods for analysing complaint data, history and frequency are discussed in section 4.3.6. If there are repeated valid complaints and non-co-operation, a council should embark on evidence gathering to take enforcement action. Complaint records and odour diaries demonstrating the adverse effect of the odour are useful in court, particularly those validated by officers.

Councils need to consider the admissibility of evidence when deciding to take enforcement action. In the sentencing notes from ARC v Nuplex, [ Auckland Regional CouncilvNuplex Industries Limited, Auckland District Court, 13 November 1998. CRN Nos 7004021704; 7004021706.] the judge commented on the issue of whether the sentencing can take into account a continuing offence. In this case, the company had pleaded guilty to two charges of odour on isolated occasions, and the charges for a continuing offence had been withdrawn. The judge considered the extent to which he could take account of the evidence of ongoing effects, since they were unproven in this case, and whether the incidents had to be assumed to have occurred in a vacuum as an isolated incident. The case suggests that one-off incidents where there is an adverse effect confirmed by a council officer may be easier to prove in court, compared to ongoing complaint records and community information.

Ongoing monitoring may require considerable council resources and/or be drawn out over many months. This can make the process frustrating for all concerned. Response times will often mean that an officer is unable to get to the site in time to validate the complaint. Proactive monitoring, which involves visiting a site frequently over a short period of time, at times when odour is expected to occur (e.g. early morning) is recommended. This has the effect of building up a better picture over a shorter timeframe and provides the ability to take appropriate action more quickly than would otherwise be possible. Sometimes the action may be to do nothing, because the council has not substantiated any odour problems and more resources are not justified.

Where a problem is acknowledged by a discharger, community liaison can be useful to work through a solution and negotiate timeframes that are realistic for all parties. If timeframes are not met, councils should consider enforcement action.

Where a community is not satisfied with the actions of the council and/or the discharger, common law actions are available (as discussed in section 1.2).

5.2 Process management and controls

This section outlines mitigation options for avoiding, reducing and managing odours at a specific site. The mitigation method that is appropriate depends on site and process requirements. Reducing odour or the potential for odour effects can be achieved using one or a combination of approaches. The appropriate solution depends on the nature of the odour, the contaminants present, the characteristics of the source and the control efficiency required. Assessment of the controls needed must consider both normal and abnormal conditions. In many cases a high level of specialist engineering input is required to develop the most appropriate solution.

5.2.1 Site design

Site planning is the key consideration for all odour sources, particularly those that are diffuse and difficult to capture and control, such as intensive agricultural activities and waste management activities. The following issues should be considered:

• the designated land use of the site and the surrounding land under the district plan
• the location of activities within the site and their orientation in relation to prevailing winds and sensitivity of the downwind receptors
• the presence of buffer distances to the site boundary and to sensitive land uses
• the need for screening, such as by earth bunds, shelter belts or natural topography.

5.2.2 Process design and management

Good design and operating procedures can prevent and minimise odour problems. They avoid or reduce the need for 'end-of-pipe' controls in some cases. The design should consider raw materials, waste handling, processing plant, instrumentation and control, and plant buildings. Process monitoring allows action to be taken to prevent system failure. It is essential to:

• select or change raw materials to reduce the potential for odours (e.g. for low VOC paints, or improving raw material quality for rendering)
• ensure odour sources are adequately enclosed and that equipment is accessible for cleaning
• incorporate and monitor process operating conditions such as temperatures and pressures that will minimise odour, and monitor parameters that are important for good performance (e.g. dissolved oxygen in oxidation ponds, pressure drop and gas temperature in a biofilter, and chemical concentration for a chemical scrubber)
• implement a preventative maintenance programme to minimise equipment failure and unplanned downtime
• educate staff about the importance of regulatory compliance and good management for achieving compliance
• have a regime of good housekeeping
• conduct odorous operations during weather conditions that are most favourable for dispersion where no other mitigation option is available (e.g. avoid early morning and evenings, consider wind direction in relation to sensitive areas, avoid hot humid weather).

Procedures and controls should be documented in a management plan. An example outline of a management plan is provided as Appendix 3.

5.2.3 Community consultation and negotiated solutions

As discussed in section 4.7, communicating with the local community is important for building a good relationship and trust, which are a useful foundation for times when odour problems do occur. It is important to bring the community on side as part of the problem-solving process, both to help identify where problems lie and to negotiate solutions, including timeframes for implementation.

Abnormal odour events can occur without warning or may be from planned maintenance. Letting people surrounding the site know about such events as early as possible helps reduce annoyance in the community. The discharger should also inform people about what is being done to remedy the problem and to prevent its recurrence, and how long the problem will take to fix. The level of annoyance may reduce if people see that the discharger is genuinely addressing adverse effects in a proactive manner.

Methods for communication include public meetings, community working parties, mail drops and a phone line for complaints and enquiries.

If the site is well operated and having very little impact, it may be difficult to get people to attend a community meeting. In these cases a less formal approach, such as an annual barbecue, can provide a useful opportunity for community feedback.

5.2.4 Control technologies

Odour sources that require treatment need to be captured and ducted to control equipment. Some gas streams require pre-treatment steps, including particulate removal and cooling or condensation to remove moisture and reduce temperature, depending on the final control option. Condensation reduces the volume of gas to be treated and therefore reduces the control equipment sizing requirements, but condensation creates a liquid waste stream. Pre-treatment may also involve humidification (e.g. prior to a biofilter). Installation of control technologies can be staged over a number of years, with gradual improvements being made as technology advances (see Case Study 3).

Odour control equipment is generally limited to a small range of technologies that have been used for many years. Most technologies are well understood and have proven performance. The available technologies are summarised below. More information on the systems and their application is provided in Appendix 5.

Dilution and dispersion

Dilution and dispersion are usually achieved via emission through a tall stack. A stack will be appropriate for very low-intensity or non-offensive odours, discharged at low rates and as a final step following treatment of an odorous gas stream. The stack should be appropriately designed to ensure it is an adequate height above buildings in the vicinity, and this may require dispersion modelling. Efflux velocity is an important consideration and there should be an unrestricted final vertical discharge (i.e. hooked vents or rain caps that restrict flow should be avoided). Dispersion has a moderate capital cost but low running costs.

Masking compounds and neutralising agents

Masking compounds and neutralising agents are products available for treating fugitive odours such as from landfill working faces, felmongeries, intensive farming of animals, and wastewater treatment plants.

The compounds are inexpensive in terms of capital cost but not many agents are well proven. They can also be expensive in the long term and certain agents should not be used for specific activities, such as certain chemicals in poultry sheds that may lead to residue issues. The amount of compound required may also make the method cost prohibitive.

The products available can be classified as follows.

• Masking agents are mixtures of aromatic oils that cover up an objectionable odour with a more desirable one. Care needs to be taken with the use of masking agents because the combination of chemicals may result in an odour that is even more objectionable or offensive.
• Chemical counteractants are mixtures of aromatic oils that cancel or neutralise odour and reduce the intensity.
• Digestive deodorants contain bacteria or enzymes that eliminate odour through biochemical digestive processes. These are usually added to wastewater treatment systems to promote biological activity and to prevent the release of the odorous compounds into air (i.e. a preventative treatment as compared to the above, which are air sprays that modify or remove the odorant once it is in the air).

Masking agents are generally only suitable for assisting in the control of odours from large-area sources, such as landfills. Even in these cases they should not be relied upon for odour control, but should act as a 'last line of defence' after stringent management practices and adequate buffer distances. Agents are often more suited to process failure or abnormal emissions than routine control. In these cases they should be seen as a temporary rather than a permanent solution.

Volatilisation of the active ingredients and contact with the odorous molecules are key to the success of air sprays. More research is required to establish the effectiveness of many of the products available, but subjectively it appears that some compounds have worked in some circumstances.

Incineration

Incineration is the destruction of odorous pollutants by thermal oxidation into carbon dioxide and water. Incineration is best applied to carbon- and hydrogen-containing odorants, such as volatile organic compounds (VOCs) and landfill gas. There are several types of incinerator or thermal oxidiser equipment design: thermal, recuperative, catalytic, regenerative and flares. Incineration has high capital and operating costs, but generally high treatment efficiencies can be achieved.

Scrubbing and adsorption systems

Wet-gas scrubbing, gas-to-gas oxidation or solid-phase systems can remove or change the chemical composition of odorous contaminants.

Wet-gas scrubbing or absorption contacts the gas with a liquid phase. The contaminant either reacts with or dissolves in the liquid and is removed in the liquid phase. The most common types of wet-gas scrubbers are packed tower or plate absorbers. Careful selection of scrubber liquors is needed, and usually involves trials. Scrubbers require regular maintenance and daily tests of active agents, and pH control in some cases. There is also a liquid waste to dispose of.

Oxidation is the most common reaction in both liquid and gas treatment methods. Oxidising agents include hypochlorite, chlorine gas, permanganate and ozone. Generally accepted practice is multi-stage chemical scrubbing or catalysed chemical scrubbing. In some applications chemical scrubbing also employs an activated carbon or adsorption stage. Gas-to-gas oxidation systems, including ozonation, are no longer widely used.

With adsorption systems, contaminants attach or condense onto the surface of an adsorbent which is a porous solid. Carbon, zeolite, bentonite and polymer adsorbents have been used to adsorb VOCs and other pollutants from relatively dilute discharge concentrations. Other adsorbents used include alumina, activated clay, silica gel and molecular sieves. Some adsorbents can be regenerated by desorption and the media used again. The compounds emitted can sometimes be recovered and reused.

Chemical additives

A range of techniques can be applied to reduce odour potential at source, including the use of certain chemical additives or stabilising agents. For wastewater treatment systems and sewers, a range of chemicals can be added to the effluent to control odour or reduce odour potential. Chemicals such as hydrogen peroxide and potassium permanganate can be directly added to oxygenate wastewaters. This technique can be useful to stabilise systems that have become anaerobic. Ferric salts and magnesium hydroxide have reportedly been added to wastewater to make the sulphur unavailable for forming odorous compounds. The techniques would typically form part of an odour control regime and would rarely be adequate on their own.

Biofilters and bioreactors

Biofiltration is where organic contaminants in a gas stream pass through a bed of material and adsorb onto the surface, where they are broken down by micro-organisms (see Case Study 4). Volatile compounds break down to carbon dioxide, water, mineral salts and other harmless products. The bed material may be soil, bark, compost or any mixture of these components. Synthetic bed materials are also used.

Bed material is contained in a structure or in a depression in the ground and the gas stream is distributed through slotted pipes or hollow pre-cast concrete blocks placed under the filter bed. Destruction efficiencies for the removal of odour can be difficult to set and monitor, because of the difficulty of measuring odours at low concentrations using olfactometry. In addition, odour of a different (non-offensive) nature can be present in the discharge from a biofilter (e.g. an earthy smell that does not relate to the compounds that were removed in the filter), but olfactometry cannot distinguish between the two in terms of odour units measured.

Bioreactors operate in a similar way to biofilters but use an inert support medium such as plastic rings, scoria or pumice. Micro-organisms are cultured as a biofilm on the surface of support media, where volatile compounds are absorbed and broken down.

Biological filtration and bioreactors are often the least-costly option for large gas volumes, and have been successful for rendering plants, wastewater treatment plants and for some VOC control. Design and operation of residence time, temperature, moisture content and nutrient balance are critical to ensure good operation of biofilters.