Capacidad analítica confia- confia-ble de dioxinas y furanos

Numerous configurations of composting technologies exist. The typical ones are presented below (Christensen, 2011). The curing time depends on the waste mix composition and applied technol- ogy.

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3.2.1 Open composting

 Windrows are elongated feedstock piles used for complete composting or just stabilization. Those are typically turned by machines for mixing but the oxygen supply happens to a greater extent by natural aeration. Retention time is 12-20 weeks for biowaste.

 Static piles provide no agitation implying need for adding bulking waste for structure. It is widely used for treating mixtures containing sewage sludge where the piles are covered with already matured compost to prevent heat loss. Some facilities may have passive aeration pipes in the base layer. Retention time is 3 weeks prior to 6-8 weeks of windrow maturing. In other types the feedstock is distributed in composting cells and moved around to ho- mogenize temperature and moisture.

3.2.2 Enclosed composting

 Channel composting happens in a hall with feedstock stacked in uncovered piles, often di- vided by walls serving as a track for turning machines or a feed-in conveyor belt for com- post material. Active aeration systems below channels and turning are used to supply air, and aeration and water addition composting processes are controlled for each channel. The compost moves along the channel during turning and the composting phases may last 6-8 weeks, or 3 weeks with a 6 month maturing in static piles.

 Aerated pile composting with turning machines happens inside a building with the feedstock placed in one large pile to be turned alongside the hall length. Forced aeration system is also installed underneath the compost bed, and usually water is added during the automatic turn- ing, collecting the off-gasses for treatment in a biofilter. Retention time is of 4-12 weeks de- pending on the turning frequency and thereby the moisture gradient.

 Brikollari is prepared by amending biowaste with bulking agent and compressing it into blocks with surface channels for natural aeration. Compressing requires electrical energy but saves space. The block stacks are conveyed to a high-rack warehouse with several ventila- tion areas, for efficient degradation and stabilization during 5-6 weeks. The stabilized com- post (20% moist) can be marketed after grinding, or further cured in windrows for 8-10 weeks.

3.2.3 Reactor composting

 Tunnel/box composting has different levels of process controls and is widely used to com- post MSW, sewage sludge, and manure. These maintain homogeneous temperature and moisture in the spacing between the compost owing to recycled exhaust gases, thus rarely needing turning. Fresh air or recycled gases are injected from below the tunnel reactors and irrigated from above, and compost is fed or removed through end hatches with conveyors. Turning for longer retention can happen within or between tunnels. Usual retention time is 1-7 weeks.

18  Rotating drum composting is a widespread, dynamic treatment of especially MSW. The

biomass is aerated by rotation mixing in a “ball mill” or an aerated fan, and the drier and homogenized compost output enables more efficient reject removal. Slow rotation ensures no compaction of wet feedstocks. Retention time is 1-10 days, where longer retention en- sures high-rate degradation, however with a necessary windrow composting lasting 2-3 months.

3.2.4 Application

In EU compost is mostly used in the agricultural sector as soil fertilizer (see Table 1). Compost low in nutrients (e.g. yard compost) is well suitable for non-agricultural applications. The bioresidual from AD may contain abundant NH3-N harmful to young roots, why this can only be applied in

non-agriculture if previously composted.

When manufacturing top soils for landscaping, cured compost is commonly refined by large screen- ers and blended with growth media to reach desired physical characteristics. For general (non- agricultural) use, plant nutrients and heavy metals along with biodegradability is normally requested declared, as the purpose is to produce quality vegetation by slow nutrient release from cured com- post rather than maximizing harvest. Compost is required well matured prior to application as to fully provide the soil with nutrients.

When using compost for backfilling larger plants, the excavated soil is mixed with compost in a 1 (nutrient-poor) to 2-3 parts soil ratio or 1 (nutrient-rich) to 4-6 parts soil, or simply applying a thickness layer of compost around the plant, depending on quality. Once compost has been mixed into the soil, dissolved plant nutrients and salinity may not be excessive for damaging plant roots. Therefore compost with lower soluble nutrient content from yard waste or sewage sludge (although often high on P content) are more adequate for landscape application compared to nutrient-rich compost from substrates such as kitchen waste and manures.

Table 4: Sectors and distribution of utilized compost within the European Union by volume (Christensen, 2011)

Application EU average (approx.) Individual EU countries

Agriculture and field horticulture 40 10-70

Landscaping, reclamation, manufactured top soils 30 20-60

Residential/private gardens 20 10-50

Others (greenhouses, nurseries, landfill cover, etc) 10 5-20

3.2.5 Environmental aspect

N losses occur during composting but are hard to quantify and data is very limited. Since most of the N in biowaste is organic, mineralization releases N as dissolved organic N and NH4+ and the

19 release of NH3 depends on N in waste, degradability, temperature and pH, and amount of gas

released through the compost as ventilation enhances H+ (acid) formation. Highly relying on the degradability and moisture content, microbes oxidize NH4+ to NO3- and intermediates appear as

NO2- (dissolved) and N2O (gas). Most pollution in general is found in leachate and if generated, N

is found in high concentrations as organic, NH4+, or NO3- bound. If not appropriately managed

compounds may percolate to environment, contributing to global warming (N2O) and soil acidifica-

tion (NO2- and NO3-).

The very potent greenhouse gasses CH4 and N2O are generated in less aerated pockets and the

formation depends on its characteristics and structure as well as technological feature of facility. It is unknown how much CH4 is oxidized prior to release and sometimes even biofilters may be a

source of e.g. N2O when NH3 volatilizes (Christensen, 2011). Complete oxidation is imperative in

terms of avoiding negative effects on plant growth from marketed compost.