Why a landfill smells and what is done with the cell surface
Odour from a landfill or waste storage site is rarely constant: it is barely noticeable on a cool morning and reaches the houses on a hot evening after rain. The operator needs to know exactly where it comes from, because an open working cell, a leachate pond and a sludge drying bed have different causes and different remedies. This article is about the waste surface: why it smells and what is done with it in daily operation.
Where the odour comes from
The main source is the decomposition of organic matter: food scraps, green waste, wet paper, textiles. While the waste has access to air, the organics oxidise to carbon dioxide and water, and the odour is weak. But waste is placed in layers and compacted, and even at a shallow depth no oxygen is left. There the organics decompose anaerobically, that is, without oxygen.
Anaerobic decomposition produces substances with a low odour threshold. Hydrogen sulphide smells of rotten eggs and appears when proteins break down and sulphates are reduced. Mercaptans and other sulphur compounds give a putrid odour that people sense at very low concentrations. Ammonia is released from nitrogen-containing organics. Volatile fatty acids give the sour "silage" smell of fresh wet waste.
The mechanism is the same as in an old treatment tank or a pressure main: where there is no oxygen, sulphate-reducing bacteria produce hydrogen sulphide. The chemistry of this process is covered in detail in Dissolved oxygen and odour at treatment plants. At a landfill the difference is the scale, and the fact that the source is an open surface of several hectares.
Why the odour comes in waves
Complaints are usually linked not to the average odour level but to peaks. They have clear causes.
Heat speeds up decomposition and the evaporation of volatile substances. After rain, water seeps into the waste mass, reactivates old anaerobic zones and pushes gas towards the surface. A drop in atmospheric pressure also draws gas out of the waste, so the odour often intensifies before bad weather. In the evening and at night the air near the ground cools and stops mixing, and the odour drifts along low ground to places where it was absent during the day.
The practical conclusion: odour has to be recorded together with the weather. A log with the date, time, wind direction, temperature and the previous day's rainfall quickly shows which conditions produce peaks and which area the odour comes from.
Working cell, leachate and sludge drying beds
A single site usually has several sources, and they are worth separating.
The working cell, where waste is delivered today, smells of fresh organics and acids. Here the open surface area decides: the smaller the area worked during the day and the sooner it is covered, the less odour.
Old cells smell of hydrogen sulphide and mercaptans: this is gas from depth escaping through cracks, settlement and thin spots in the cover. Here the integrity of the cover matters, and at large sites gas collection.
Leachate (liquid that has passed through the waste mass) in open ponds and manholes is a separate strong source. It is rich in organics and ammonium, and in a stagnant pond it quickly turns anaerobic. The less rainwater enters the waste, the less leachate forms. What a field trial of a sorption medium on leachate showed is described in Landfill leachate.
Sludge drying beds at treatment plants smell for the same reason: wet, organic-rich sludge lies in the open for a long time. The longer and thinner the layer dries in the air, the longer it smells. Mechanical dewatering reduces the sludge volume and its open storage time; the equipment is in the sludge dewatering section.
What is done with the cell surface
The classic measure is daily cover of the working cell with soil. It cuts the waste off from air, birds and wind. The weak points of soil cover are well known: soil takes up useful cell volume, has to be hauled, dries out and dusts in hot weather and washes away in rain. Soil by itself does not bind odour; it only covers the surface.
Fragrances and spraying "neutralisers" along the perimeter do not remove the source. They change the character of the odour while the sprayer runs and do not help once the wind shifts.
An intermediate option between soil and complex systems is an active cover: a thin layer of material that not only covers the surface but also sorbs part of the odorous substances, evens out moisture and holds light fractions in place. An example of such a development is EcoSorbent. The effect of any cover depends on the waste composition, moisture and weather, so carrying a result over from one site to another proves nothing. The decision is made on a pilot area.
A pilot makes sense only if the result can be compared. To do this, the initial state is recorded before application, and then the same area is compared with an adjacent untreated one in the same weather.
On old cells, where odour comes from depth, surface measures work less well. Gas finds its way through any weak spot in the cover, so cracks and settlement are repaired first, and at large sites gas collection wells are designed. The collected gas is flared or used, and then it stops escaping through the surface. This is a capital solution, decided on the basis of a survey, not on one week of complaints.
Initial assessment checklist
This data is needed before choosing any measure: it shows which source is the main one and where to start.
- ✓Type of site and area of each zone: working cell, old cells, leachate ponds, sludge drying beds
- ✓Share of organics in incoming waste
- ✓Where and when the odour is strongest: time of day, weather, wind direction
- ✓Distance to the nearest houses, roads and recreation areas
- ✓What the working cell is currently covered with, and how often
- ✓Cracks, settlement and uncovered areas on old cells
- ✓How leachate is collected and where it is stored
- ✓Dust and wind-blown light fractions
- ✓A complaints log with dates and weather for recent months
- ✓Photos of problem areas
What next
Start with an odour log and a site plan marking all open surfaces. After a few weeks of records it will be clear which area produces the peaks and in what weather. If the main source is leachate or sludge drying beds, work on them rather than on the cells.
If the main source is the waste surface, first reduce the open area of the working cell and check the integrity of the cover on old cells. Then choose a pilot area for an active cover and agree in advance how you will compare the result. For sludge drying beds, assess whether their area can be reduced by mechanical dewatering: the options are in the sludge dewatering section, and wastewater treatment lines in the industrial wastewater treatment section.