most clean and source-separated biodegradable waste streams, the levels of ptes will be low. in the case of wastes originating from industry, the levels of ptes may be higher and will need to be taken into consideration when determining a suitable end use for the material. digestate typically contains elements, such as lignin, that cannot be broken down by the anaerobic microorganisms. also, the digestate may contain ammonia that is phytotoxic, and may hamper the growth of plants if it is used as a soil-improving material. for these two reasons, a maturation or composting stage may be employed after digestion. lignin and other materials are available for degradation by aerobic microorganisms, such as fungi, helping reduce the overall volume of the material for transport. during this maturation, the ammonia will be oxidized into nitrates, improving the fertility of the material and making it more suitable as a soil improver. large composting stages are typically used by dry anaerobic digestion technologies. wastewater the final output from anaerobic digestion systems is water, which originates both from the moisture content of the original waste that was treated and water produced during the microbial reactions in the digestion systems. this water may be released from the dewatering of the digestate or may be implicitly separate from the digestate. the wastewater exiting the anaerobic digestion facility will typically have elevated levels of biochemical oxygen demand (bod) and chemical oxygen demand (cod). these measures of the reactivity of the effluent indicate an ability to pollute. some of this material is termed 'hard cod', meaning it cannot be accessed by the anaerobic bacteria for conversion into biogas. if this effluent were put directly into watercourses, it would negatively affect them by causing eutrophication. as such, further treatment of the wastewater is often required. this treatment will typically be an oxidation stage wherein air is passed through the water in a sequencing batch reactors or reverse osmosis unit. history gas street lamp the history of anaerobic digestion is a long one, beginning as early as tenth century bce in assyria where biogas was used to heat bath water. reported scientific interest in the manufacturing of gas produced by the natural decomposition of organic matter dates from the 17th century, when robert boyle (1627-1691) and stephen hales (1677-1761) noted that disturbing the sediment of streams and lakes released flammable gas. in 1778, the italian physicist alessandro volta (1745-1827), the father of electrochemistry, scientifically identified that gas as methane. in 1808 sir humphry davy proved the presence of methane in the gases produced by cattle manure. the first known anaerobic digester was built in 1859 at a leper colony in bombay in india. in 1895, the technology was developed in exeter, england, where a septic tank was used to generate gas for the sewer gas destructor lamp, a type of gas lighting. also in england, in 1904, the first dual-purpose tank for both sedimentation and sludge treatment was installed in hampton, london. imhoff tank by the early 20th century, anaerobic digestion systems began to resemble the technology as it appears today. in 1906, karl imhoff created the imhoff tank; an early form of anaerobic digester and model wastewater treatment system throughout the early 20th century. after 1920, closed tank systems began to replace the previously common use of anaerobic lagoons – covered earthen basins used to treat volatile solids. research on anaerobic digestion began in earnest in the 1930s. around the time of world war i, production from biofuels slowed as petroleum production increased and its uses were identified. while fuel shortages during world war ii re-popularized anaerobic digestion, interest in the technology decreased again after the war ended. similarly, the 1970s energy crisis sparked interest in anaerobic digestion. in addition to high energy prices, factors affecting the adoption of anaerobic digestion systems include receptivity to innovation, pollution penalties, policy incentives, and the availability of subsidies and funding opportunities. modern geographical distribution today, anaerobic digesters are commonly found alongside farms to reduce nitrogen run-off from manure, or wastewater treatment facilities to reduce the costs of sludge disposal. agricultural anaerobic digestion for energy production has become most popular in germany, where there were 8,625 digesters in 2014. in the united kingdom, there were 259 facilities by 2014, and 500 projects planned to become operational by 2019. in the united states, there were 191 operational plants across 34 states in 2012. policy may explain why adoption rates are so different across these countries. feed-in tariffs in germany were enacted in 1991, also known as fit, providing long-term contracts compensating investments in renewable energy generation. consequently, between 1991 and 1998 the number of anaerobic digester plants in germany grew from 20 to 517. in the late 1990s, energy prices in germany varied and investors became unsure of the market's potential. the german government responded by amending fit four times between 2000 and 2011, increasing tariffs and improving the profitability of anaerobic digestion, and resulting in reliable returns for biogas production and continued high adoption rates across the country. incidents involving digesters anaerobic digesters have caused fish kills (e.g. river mole, devon, river teifi, afon llynfi, and loss of human life (e.g. avonmouth explosion) there have been explosions of ads in the us (jay, maine pixelle specialty solutions’ androscoggin mill; pensacola (cantonment) 22 january 2017 (kamyr digester explosion); epdm failure march 2013 aumsville, oregon; february 6, 1987, pennsylvania two workers at a wastewater treatment plant were re-draining a sewage digester when an explosion lifted the 30-ton floating cover, killing both workers instantly; southwest wastewater treatment plant in springfield, missouri), in the uk (for example at avonmouth and at harper adams college, newport, shropshire), plus in europe, there were about 800 accidents on biogas plants between 2005 and 2015, e.g. in france (saint-fargeau) (though few of them were 'serious' with direct consequences for the human population). fortunately, according to one source, 'less than a dozen of them had consequences on humans'- for example, the incident at rhadereistedt, germany (4 dead). safety analyses have included a 2016 study compiled a database of 169 accidents involving ads.