ODOURS

Odours are chemicals that stimulate our sense of smell. We can only perceive odours as we breathe in air, bringing the molecules into contact with the olfactory mucous membranes present in the nasal cavities.

We mainly perceive odours in two ways:

–        through a “physiological” process that occurs when the receptors in the nose are stimulated by chemicals with an odour and transmit electrical stimuli to the brain, which decodes them by associating them with known substances, through memory;

–        through a “psychological” process that is activated by the experiences, memories and emotions that each we associate with a certain odour.

When they perceive an odour, most people are not able to have precise information about its components or about the different chemicals that are part of it and characterize it.

The overall sensation of odour does not in itself correspond to a predetermined physical quantity, but rather depends on the subjectivity of the individual.

 

 

PERCEPTION and SENSATION

Technically, when we talk about ‘perception’ and ‘sensation’, we are referring to two different concepts:

–        by ‘perception’ we mean detecting an odour at neuronal level;

–        by ‘sensation’ we mean the act of becoming aware of the detection of an odour (between the biological recording of an odour and the awareness of its perception there is a time interval of about 500 milliseconds).

 

 

ODOUR EMISSIONS and OLFACTORY NUISANCE

The complex olfactory structure in humans is intrinsically subjective, so not all perceptible and perceived odours can be considered ‘olfactory nuisances’ (also because potentially and in the presence of certain factors [e.g. intensity, duration, frequency, range time, context of the emission etc…] any odour is capable of generating an ‘olfactory nuisance’). For this reason, the term is associated with a capability to generate negative effects (e.g. following extended and repeated exposure over time).

Unpleasant odour emissions that come from industrial and agricultural activities, and in particular those dealt with herein, from waste management activities on landfill sites, represent a problem that involves many people and has a significant impact on the quality of life of exposed subjects. Assessing the legitimacy of odour emissions is an extremely complex factor that involves criteria relating to tolerability and subjective sensation, as well those concerning health and safety and environmental protection.

In Italy, ‘olfactory nuisance’ also falls within the field of criminal law ([1]).

 

 

THE EN 13725:2003 STANDARD and DYNAMIC OLFACTOMETRY

Europe regulated the measurement of odour concentration with the EN 13725:2003 standard, which defines a method for the objective determination of the odour concentration of a gaseous sample using dynamic olfactometry with human assessors and the emission rate of odours emanating from point sources or area sources.

We are not, however, aware of any limit values or odour thresholds having yet been regulated at European level, nor of there being specific EU provisions to define the legitimacy of “unpleasant” odour emissions.

Despite this, in order to be able to operate, manufacturing businesses must also consider the odour-generating impact they cause and this impact must be objective, measurable and quantifiable and therefore considered “legitimately tolerable”.

Precisely for this reason, from right back in the design phase, it is necessary to consider implementing actions to prevent the formation and diffusion of odour emissions, and it is also necessary to adopt management rules that can limit the release of these substances as much as possible.

 

At national regulatory level, the Italian UNI 11761:2019 standard was issued which is not intended to be applied to the use of monitoring tools for the purpose of protecting health and safety in the workplace, but aims to indicate the technical management requirements of the automatic systems used for odour monitoring (IOMS, Instrumental Odour Monitoring Systems) as well as the criteria for the periodic measurement of odours found at sites where meters are installed.

 

 

EMISSIONS and IMMISSIONS

So far, we have spoken about emissions, but what are the differences between an “emission” and an “immission”? For information purposes only and in an extremely simplified way, we can say that:

–        “immission” means the substance is introduced into the atmosphere; it is measured on the basis of volume flow, temperature and speed.

–        “emission” means the concentration of certain substances in the air; the concentration is measured by a detection station at the receptor and is determined on the basis of the quantity present in units of volume.

 

Article 269 of Italian Legislative Decree 152/2006 states that plants that produce emissions into the atmosphere must request environmental authorisation from the competent bodies.

 

 

DEFINITION of ODOUR EMISSIONS

The definition is referred to in Italian Legislative Decree no. 102 of 30 July 2020 which introduced to Article 272-bis of Italian Legislative Decree 152/2006 the notion of “odour emissions”, defined as “emissions released or diffused with odour-generating effects” (Article 268, paragraph 1, letter f-bis).

 

 

LANDFILL ODOUR EMISSIONS

In landfills, we talk above all about emissions, which can correspond to:

  • point sources (we talk about “point sources” when the odour is emitted from a single point, generally in a controlled way through a chimney);
  • area sources (we talk about “area sources” when the emissions occur from fairly extensive solid or liquid surface areas. Area emissive surfaces can be divided up into those with an induced or active flow (e.g. sources with an outgoing air flow such as biofilters) and those without an induced or passive flow (when the only flow present is that due to the transfer of matter from the surface to the overlying air e.g. landfills, waste water treatment plant tanks));
  • and in some specific cases also volume sources (we talk about “volume sources” when emissions occur from buildings, either intentionally through naturally ventilated ducts, or unintentionally through doors, windows or other openings).

The main flows of landfill odour emissions may be “conveyed flows” (e.g. in supply pipes, wells, chimneys), “free or dispersed flows” from extensive surface areas with natural ventilation (e.g. landfill surfaces in the cultivation phase, in the temporary cover phase, in the permanent cover phase), “discharges from fugitive sources” with a non-measurable volumetric flow rate (e.g. wells in the drilling phase or in the wellhead raising phase) and, finally, “ambient air from processing rooms”.

The method for measuring odour and the technique used for sampling are key. In particular, the sampling technique depends on the type of source (Gostelow et al., 2003; Bockreis and Steinberg, 2005).

 

Technical literature ([2]) emphasizes that the estimated OER ([3]) (“Odour Emission Rate” expressed in odorimetric units per second (ouE/s) and obtained as the product of the odour concentration for the gas flow) for passive area sources is rather complicated, as it is difficult to measure a representative odour concentration and above all to determine a well-defined air flow. Furthermore, the anisotropy of the mass of waste that has built up over the years (with reference to the type and stratification, density, gas permeability, type and efficiency of drainage systems, conveyance and the set of collection systems present in the landfill body, the extent of the pressure gradient applied to the collection systems etc.) makes it difficult to determine significant sampling points for measuring the flows of real odour emissions from the landfill body. Once produced, landfill biogas actually tends to migrate naturally through the mass of waste via preferential routes which vary over time and are therefore not always known.

 

This topic certainly deserves some consideration regarding the dispersion of emissions into the atmosphere and the related dispersion models, but we will leave it up to you to explore this decidedly complex issue.

 

WHAT IS A LANDFILL and WHY DOES IT SMELL?

Article 2(g) of Directive 1999/31/EC defines a landfill as “a waste disposal site for the deposit of the waste onto or into land (i.e. underground)…”.

 

With the new Directive (EU) 2018/850 (which amends Directive 1999/31/EC) two important objectives have been introduced that European member countries will have to achieve in the coming years:

  • as of 2030, all waste suitable for recycling or other recovery shall not be accepted in a landfill;
  • by 2035, the amount of municipal waste landfilled shall be reduced to 10% or less of the total amount of municipal waste generated.

But this measure will not completely solve the problem of odour emissions from landfills, which will nevertheless remain an aspect to be managed.

“Bad smells” from landfill sites come from leachate but above all from biogas generated by natural waste degradation and decomposition processes.

 

 

PRODUCTIVITY TIMES and PRODUCTION PHENOMENOLOGY OF LANDFILL BIOGAS

The decomposition of waste in landfills takes place by means of various processes featuring various, complex aspects that we will set out in a highly simplified way below:

  • physical degradation involves the modification of the physical characteristics of the waste itself (e.g. the reduction in volume and the precipitation, release and absorption of substances);
  • chemical degradation involves the activation of reactions between the various substances making up the waste (e.g. in leachates: variation of pH, redox potential, solubility);
  • biological degradation involves the transformation of the material that constitutes the waste by microorganisms such as bacteria and takes place mainly in three stages (aerobic stage, anaerobic stage and anaerobic methanogenic stage) by means of which the biogasification process of the waste occurs.

We’ll now touch on the biogas production phenomenology, which can be summarized in these three main stages:

  • the aerobic stage: this occurs immediately after the waste is deposited, the work of aerobic microorganisms, and depends on the availability of oxygen present in the matrix. It is usually short-lived (from a few hours to a few months) and is linked to the type of waste. The aerobic process is significantly exothermic (heat production that can reach temperatures of 70 °C) and is characterized by emissions of carbon dioxide, water and partially degraded organic substances.
  • the anaerobic (acidic) stage: this occurs when the availability of oxygen is reduced to the point where an aerobic process is no longer possible. In this context, aerobic microorganisms prefer to use free oxygen but, in its absence, they can use bound oxygen. In this stage, the production of carbon dioxide takes place. There is less generation of thermal energy compared to the aerobic process and a considerable production of partially degraded organic substance.
  • the anaerobic methanogenic stage (consisting in turn of the ‘non-stationary anaerobic methanogenic stage’ and the ‘stationary anaerobic methanogenic stage’): in this stage, the microorganisms convert the organic substance partially degraded by the aerobic organisms into methane, carbon dioxide and other micro-components. The methanogenic stage occurs after a period that varies between 3-6 and 9-12 months from the disposal of the waste in the landfill.

 

Once the methanogenic stage has begun, biogas is produced for many years (over 30), according to a trend that highlights the maximum production in the first years and a progressive asymptotic exhaustion until the organic substance is completely degraded or for as long as there are the environmental conditions suitable for the process. This means that even post-closure landfills can contribute to the production of biogas and that the organic fraction of waste brought to landfills today has the potential to produce biogas – and therefore “bad smells” – until at least 2051.

 

STANDARD PARAMETERS CHARACTERIZING ‘TYPICAL’ BIOGAS AND BIOGAS ODOUR CHARACTERISTICS

Landfill biogas (landfill gas – LFG) is therefore a mixture of gases consisting mainly of methane, carbon dioxide and other micro-components in variable percentages.

The volumetric incidences of the various gases present in landfill biogas are so variable that technical literature in the sector has developed the references set out below ([4]):

 

COMPONENT GAS STANDARD INCIDENCE
Methane 0 – 60%
Carbon dioxide 0 – 70%
Oxygen 0 – 21%
Nitrogen 0 – 79%
Hydrogen 0 – 1%
Water 0 – 5%
Hydrogen sulphide 0 – 2%
Ammonia 0 – 1%
Carbon monoxide   0 – 0.1%

 

Although methane, carbon dioxide, oxygen and nitrogen consistently characterize biogas, it is the micro-component gases that provide the mixture with its distinctive odour characteristics.

 

Technical literature also defines the standard parameters characterizing ‘typical’ biogas:

STANDARD BIOGAS PARAMETER UM STANDARD VALUE
Volumetric unit of measurement Nm3
Composition
Methane CH4 % vol 50
Carbon dioxide CO2 % vol 35
Oxygen O2 % vol 3
Nitrogen N2 % vol 12
Micro-components traces
Heating value

(proportional to the concentration of methane)

 kWh 4.79
Flammability limits in air
Lower (LEL) % in air 5
Upper (UEL) % in air 15
Density
absolute kg/Nm3 1.4
relative to air kg/Nm3 0.96
Temperature °C 40
Pressure
absolute hPa 1013 (sea level)
relative hPa 0
Humidity % RH 100

 

THE BIOGAS CLIMATE-CHANGING FACTOR

LFG 50 biogas, also called “typical” or “standard” biogas, is shown to contain a volumetric percentage of methane equal to 50% and a volumetric percentage of carbon dioxide equal to 35%.

 

Methane gas and carbon dioxide are not only the major component gases of landfill biogas, but are also considered to be among the greenhouse gases with the greatest climate changing impact.

 

In particular, the latest evaluation report of the Intergovernmental Panel on Climate Change– the IPCC – called “AR6” (published in August 2021) shows how METHANE:

  • considering a time horizon of 20 YEARS, is characterized by a GWP([5]) 2 times greater
  • considering a time horizon of 100 YEARS, is characterized by a GWP(5)9 times greater

than that of CARBON DIOXIDE.

 

This means that if the landfill biogas is not collected correctly and efficiently, in addition to generating odour emissions, it also determines GHG emissions into the atmosphere.

 

 

The latest and most recent publication entitled “What a Waste 2.0”, from the World Bank Group (2018 edition) states that:

 

I.               municipal waste landfills around the world are large producers of biogas as normally 30 to 50% of the municipal waste is composed of organic material (Figure 1)

Figure 1:

Global waste distribution by type

(source: World Bank, 2018)

 

  1. as regards waste treatment, globally, about 60% of waste is disposed of in landfills (Figure 2). Of these landfill plants, less than 8% are equipped with gas collection and capture systems; and 33% of the waste is still dumped “openly”.

Figure 2:

Global waste treatment and disposal

(source: World Bank, 2018)

III.            From the data shown in Figure 3, it appears that the adoption of more sustainable waste management methods goes hand in hand with economic development in each country. Typically, the construction and use of landfills is the first step towards more sustainable waste management.

 Figure 3: Waste treatment methods by region

(source: World Bank, 2018)

In view of these data, the survey of the ISPRA Environmental Data Yearbook (no. 89/2020) shows that the waste sector – and in particular that of landfills – represents the second greatest national source of emission of methane into the atmosphere, with a contribution equal to almost one third of all the sources identified (ref. – Table 7.4 and Figure 7.4.b of the Yearbook no. 89/2020 – ISPRA)

 

BIOGAS IS A SOURCE OF RENEWABLE ENERGY

If the landfill biogas is not collected correctly, it is released into the atmosphere and can lead to problems in addition to the odour-generating and climate-altering ones described so far. It can cause risks of fire and explosion, asphyxiation, intoxication, phytotoxicity, etc. And all these problems can manifest themselves together, increasing the overall risks for human health and the environment.

It is therefore clear how important the effectiveness and efficiency of biogas collection systems are.

Furthermore, the collected biogas can be sent for energy recovery for the production of energy from renewable sources (Directive (EU) 2018/2001 – Article 2(1)).

 

Essentially, landfill biogas collection systems can contribute to increasing the level of energy savings and can limit the odour-generating and climatic-altering impact caused by landfills.

 

COLLECTION ELEMENTS in LANDFILLS

Most of the collection elements present in landfills are actually ‘static’ collection units that have performance capabilities related to their specific product characteristics and are not capable of actively acting to make themselves more or less efficient (e.g. slotted probes).

 

On the contrary, elements such as intake units and collection automation systems are units that act actively and have the potential to make a difference in terms of the efficiency of the entire collection system present in the landfill.

 

The intake unit applies a specific pressure gradient (negative pressure or pressure drop to exercise intake) to all main transport lines ([6]) connected to it, failing however to differentiate this parameter for each individual secondary line ([7]).

 

Nevertheless, in order to prevent the odour-generating components of the biogas from dispersing into the atmosphere, the performance of the static collection units appears not to be enough. Sometimes, even the performance levels of the intake units that operate set on a single applied pressure gradient are insufficient, failing to differentiate the pressure gradients that would be required for each well.

 

Today, there are certain new collection automation systems that also regulate the individual pressure drops applied to each secondary line (7) as well as to the regulation of the parameters that characterize the collected biogas mixture (e.g. CH4, O2) in each well connected to the regulation station.

THE NEW COLLECTION AUTOMATION SYSTEMS

In recent years, various solutions have been developed and adopted to significantly improve the efficiency of traditional collection systems. GAS STABILIZER, an innovative system with an international patent (PCT no. WO2017/081671A1 – “System and method to control a biogas collection plant”), which acts as the mind/brain for the entire collection system present in the landfill, is one of these solutions.

 

Compared to traditional systems, GAS STABILIZER is positioned between element 3. (regulation stations) and element 4. (secondary biogas transport lines) and can be applied to any type of biogas collector/regulation station, on both new and existing plants.

  1. INTAKE UNIT;
  2. MAIN BIOGAS TRANSPORT LINES;
  3. REGULATION STATIONS, BIOGAS COLLECTOR PLANTS;
  4. SECONDARY BIOGAS TRANSPORT LINES;
  5. COLLECTION SYSTEMS PRESENT IN THE LANDFILL BODY

The functional diagram below identifies the main components of the GAS STABILIZER:

 

A BIOGAS EXTRACTION
B SAMPLE EXTRACTION FOR GAS TRAIN ANALYSIS
C BIOGAS ANALYZER
D DATA ANALYSIS AND CONTROL FRAMEWORK
E REGULATION VALVE
F COLLECTOR GAS FLOW
G GAS FLOW TO THE ENGINES

Two GAS STABILIZER machines are in operation at the landfill in Fano (PU – Italy)

CASE HISTORY

 

Compared to traditional landfill biogas collection systems, GAS STABILIZER brings numerous benefits, including:BENEFITS of GAS STABILIZER 

  • the overall average increase in biogas collection, in volumetric terms, equal to 31.98%;
  • the reduction, through to the elimination, of odour emissions (also in the waste disposal phase);
  • the overall average increase in the lower calorific value of the biogas collected (and sent for energy recovery);
  • the increase in the production of electricity;
  • which enables the verification, monitoring and continuous 24/7 control, in real time and remotely, for each collection line [main (6) and secondary (7)];
  • which analyzes, controls and automatically adjusts the collection parameters with frequencies set at least every 30 minutes, continuously 24/7, in real time and remotely for each collection line;
  • which keeps the set collection values constant over time thanks to the automatic continuous analysis system of the biogas taken in;
  • it can be applied to any type of collection system/regulation station;
  • it is modular, light, reusable, easy to install and can be dismantled;
  • it has low levels of consumption and can work with a photovoltaic generative island.

GAS STABILIZER also contributes to reducing the carbon footprint and to achieving the ecological transition goals of national and European policy.

([1]) ref. Article 674 of the Italian Criminal Code on the subject of the “dangerous disposal of objects”, as well as on the subject of immissions pursuant to Article 844 of the Italian Civil Code for the strictly civil aspects.

([2]) for example the ARPA FVG 2017 Procedure for the assessment of the odour impact

([3]) OER – “Odour Emission Rate” expressed in odorimetric units per second (ouE/s) and obtained as the product of the odour concentration and the gas flow rate.

([4]) EPC Libri “Biogas da discarica” by Enrico Magnano

([5]) GWP = Global Warming Potential

([6]) the main transport lines are lines that connect the regulation station to the intake unit itself

([7]) the secondary transport lines are lines that connect the individual wells present in the landfill body with the regulation station