Terra Preta Sanitation

Terra preta is the name for the black and highly fertile soil that was produced by the pre-Colombian Amazonian cultures. Terra preta soils, literally “black earth”, contain higher concentrations of nutrients (such as nitrogen, phosphorus, potassium and calcium) and stable soil organic matter (SOM) than surrounding soils (GLASER et al. 2001).

According to archaeologists, pre-Columbian native populations generated them by incorporating large amounts of charred residues (charcoal) either produced intentionally or as a by-product from human activity into the soil together with nutrient-rich material (GLASER 2006). Nutrient-rich material was used to be human and animal manure (rich in P and N), waste including mammal and fish bones (rich in P and Ca), ash residues of incomplete combustions (rich in Ca, Mg, K, P and charcoal) or plant biomass (e.g. kitchen wastes, compost, algae etc.) (GLASER 2006). The addition of charcoal resulted in high contents of soil organic matter (SOM).

The black carbon content of terra preta soils can be up to 70 times higher than surrounding, mostly infertile, soils (GLASER et al. 2001), which gives these soils their black appearance and their amazing properties. Due to its polycyclic aromatic structure, black carbon is chemically and microbiologically stable and persists in the environment over centuries (GLASER et al 2001). Terra preta soils thus act as a long-term carbon sink (GUENTHER 2007).

A hectare of meter-deep terra preta can contain 250 tons of carbon as opposed to 100 tones in unimproved soil (GLASER et al. 2001; LEHMANN et al. 2006). Black carbon can be produced either by pyrolysis of biomass or by an enzymatic conversion of some microorganism (RECKIN 2010). It has a porous structure, which is good for the structure of the soil (water retention, aeration), the fertility of the soil and it increases growth rates and abundance of certain microbes, while abundance of fungi and germs is reduced:

Over the time, the black carbon is partly oxidised producing chemical molecules on its surface (carboxylic groups), which act as a trap for nutrients, preventing them from being washed out (GLASER et al. 2001). The nutrients entrapped like this in the porous structure of the charcoal provide necessary elements for plant growth, but also food and a safe housing to bacteria which are beneficial for the soil and plants (O'GRADY & RUSCH 2007, KIM et al. 2007).

Today, it is argued that the generation of new terra preta (‘terra preta nova’) could be the basis for sustainable agriculture in the twenty-first century to produce food for billions of people, and could lead to attaining some major Millennium Development Goals: (i) to combat desertification, (ii) to sequester atmospheric CO₂ in the long term, and (iii) to maintain biodiversity hotspots such as tropical rainforests (GLASER 2006).

This idea has been taken up in the recent years from sanitation experts, as terra preta seems to be a re-discovered, but today very promising response to the challenge of sustainable and safe treatment of human excreta and domestic organic waste.

Results from the terra preta sanitation (TPS) lab scale and pilot scale experiments have shown that it is possible to convert bio-waste and faecal matter in a hygienic and sustainable way into highly fertile humus-like material (OTTERPOHL 2009, FACTURA et al. 2009).

The first step in terra preta sanitation (TPS) is lactic acid fermentation (or lacto-fermentation), followed by a second step of vermicomposting (OTTERPOHL 2009). Charcoal can be added either before or after the lacto-fermentation process.

Vermicomposting is an established process for treatment of organic solid wastes or faecal matter (FACTURA et al. 2001), but the initial silage process makes the combination superior than other combined processes. This is because the flora (microbial community) of human faeces is not really compatible with soil metabolism and the microbial mix used for lacto-fermentation aids the transition to the soil life (RECKIN 2010). Lactic acid fermentation is used throughout the world in industry to preserve foods. When the acidity rises due to lactic acid fermenting organisms, many other pathogenic microorganisms are killed (http://en.wikipedia.org/wiki/Lactic_acid_fermentation). Yogurt and sauerkraut are some of the most famous products produced by lacto-fermentation and it is the lactic acid, which gives these products their slightly sour taste.

Up to date TPS has been scientifically established for kitchen and garden wastes, human faeces and human urine (OTTERPOHL 2009; OTTERPOHL n.y. a; OTTERPOHL n.y. b; FACTURA et al. 2010). Kitchen- and garden wastes are sufficient to produce the rich vermicompost to be introduced into the soil (SCHEUB 2010), but as a sanitation system, however, the co-treatment with lacto-fermented toilet products is perceived as sustainable sanitation solution (see also the terra preta toilet factsheet). Up scaling of TPS and the application to sieved blackwater, faecal sludge, sewage sludge or organics waste may be developed in the future.

Lacto-fermentation and biological mix

Lacto-fermentation is similar to the silage production process in agriculture (OTTERPOHL 2009). It is a biological anaerobic process (like anaerobic digestion), but no gas is produced (OTTERPOHL 2009). However, lactic acid bacteria have the advantage not to be sensitive to O2 (they are aerotolerant) allowing them to grow in its presence as well as in its absence (MADIGAN et al. 2003). During lacto-fermentation sugars such as glucose, fructose, and sucrose, are converted into cellular energy and the metabolic by-product lactate (http://en.wikipedia.org/wiki/Lactic_acid_fermentation). The two most important subgroups of the lacto-fermentation and lactic acid bacteria lies in the nature of the products formed from the fermentation of sugars: One group, called homofermentative, produces a single fermentation product, lactic acid, whereas the other group, called heterofermentative, produces other products, mainly ethanol and CO₂ as well as lactate (MADIGAN et al 2003).

The main advantages of lacto-fermentation are that it works efficiently and stable without air exchange; produces no offensive odours; and prevents losses of nutrients or carbon to the atmosphere. Especially the treatment of urine by lacto-fermentation is interesting as the controlled metabolisation prevents the hydrolysis of the urea and its transformation to volatile ammonia and CO₂, which would result in the loss of nitrogen and CO₂ into the atmosphere and bad odour (RECKIN 2010; GENSCH 2010 b). 

The effective mixture of microorganisms needed for the lacto-fermenting process is available commercially. But the mix is often expensive and spoils after some weeks as they are manipulated for commercialisation. The easiest way to obtain an effective microbial mix is probably to take an inoculum from Sauerkraut (pickled sour cabbage) liquid (OTTERPOHL 2009; FACTURA et al. 2010). Dr. Reckin, a German researcher, who is involved since the beginning in TPS has developed a microbe mixture similar to the so-called Effective Microorganisms (EM), containing a mixture of five microbes namely: Bacillus subtilis, Bacillus mesentericus, Geobacillus stearothermophilus, Azotobacter croococcum and Lactobacillus sp., respectively. This mixture is specifically designed to prepare for vermicomposting and thus may be better suited than commercial effective microorganism or Sauerkraut liquor. The mix is available with him, at the Technical University Hamburg-Harburg (TUHH, Institute of Wastewater Management and Protection, aww, Germany) or from the Xavier University SuSan Center (Philippines).

The recently developed urine diversion terra preta toilets (see also terra preta toilet factsheet) are based on the separate collection and treatment of faeces and urine: The urine is collected in a Jerrican that contains the microbial mix (about half a litre of microbial mix per 20 litre Jerrican) and faeces fall by gravity into a bucket that is placed airtight underneath the toilet. The faeces are then covered with the charcoal mix in order to remove the bad odour of the faeces and providing stable organic matter for the terra preta soil production (OTTERPOHL n.y. b). The mix should contain charcoal (black carbon), stone dust, finely cut wood (or another bulking agent and to raise the C/N ratio necessary for vermicomposting) and some limestone or volcanic soil (FACTURA et al. 2010; GENSCH 2010). The microbial mix for lacto-fermentation is either added together with the ash after each defecation or after the bucket is full and removed from the toilet.

In TPS systems, lacto-fermentation should be done for 4 weeks minimum before vermicomposting (OTTERPOHL n.y. b; FACTURA et al 2010).

Vermicomposting

Vermicomposting is an aerobic decomposition process involving the combined action of earthworms and microorganisms. In TPS, lacto-fermented urine, faeces and kitchen- and garden wastes (optionally lacto-fermented) can all be vermin-composted together either in a closed composting chamber (Rottebehälter) or in open vermicompost heaps. If the charcoal mixture hasn’t been added yet, it can be added now to the compost chamber or heap. The addition of charcoal is very important as it is the key factor for the black carbon content in terra preta soils, the soil structure (air, water retention), its fertility and the abundance of microbial life.

In the case of existing composting pits or heaps, worms might not need to be added as the new material is just spread over the existing compost. For a new vermicompost, worms need to be added to the material and acclimated. Once installed, the worm population is self-regulating and will increase to the point where available food and space constrain further expansion. The digestion of the earthworms accelerates the decomposition of organic matter in comparison to conventional composting (FACTURA et al. 2010).When you dig in the heaps, you will see big healthy worms (doing vermicomposting process) transforming the bio-wastes into the nutrient-rich terra preta (OTTERPOHL n.y. b). The earthworm castings (the material produced from the digestive tracts of worms) transform the nitrogen (N), phosphorus (P) and potassium (K) and the organic matter into humic acids and other matter, which improves the fertility and structure of the soil.

Both lactic acid fermentation and vermicomposting need to be monitored but are not labour-intensive. For vermicomposting, the main parameters to control are moisture content and temperature.

The microbial mix can be kept at the household level and continuously fed and cultivated. Generally the bacteria can be duplicated every 2 days, but they should be fed at least once a week with some water, a sugar source (blended bananas, sugar can juice etc.) and sterilized milk (GENSCH 2010 b). Sugar content in the solution should be around 3 to 5 % (GENSCH 2010 b). The entire solution should be cooked for sterilization and cooled down before added (GENSCH 2010 b). Bottles, containing the microbial mix have to be clearly labelled and kept away from children.

The lacto-fermentation process should be maintained at least for one month (up to six month, OTERRPOHL n.y. a; FACTURA et al. 2010) in a completely close container before transferring the products to the vermicomposting, where they should stay for three to six month (OTTERPOHL n.y. a; FACTURA et al. 2010).

TPS is, even though rediscovered from ancient cultures, a rather new technology and still some research is needed for exact estimation of pathogen removal. However, lacto-fermentation is used throughout the world in industry to preserve foods (pathogenic microorganisms are killed when acidity rises) and vermicomposting is a rather well established treatment of kitchen and garden wastes. A recent publication indicates that lacto-fermentation followed by vermicomposting can also be effective for excreta sanitation and leads to over 99% reduction of faecal indicator bacteria in faecal matter (FACTURA et al. 2010). The effect on other more persistent pathogens in faeces like helminth ova (Ascaris lumbricoides) still requires further research.