NOVELTY - Production of biogas from waste involves subjecting the waste to biological treatment under anaerobic conditions to produce biogas and digestate; and adjusting pH of the digestate to an alkaline pH to produce uncharged ammonia in the digestate for a time to eliminate or significantly reduce the concentration of pathogens in the digestate.    USE - For production of biogas from waste, such as municipal, industrial and agricultural organic waste, including food processing waste (claimed).    ADVANTAGE - The sanitization allows shorter hold-times, reduced consumption of the pH adjusting substance, e.g. urea, and a smaller volume of the sanitization reactor. Further, reducing the volume of the sanitization reactor simplifies the heating of the same, which in turn makes it possible to use a smaller volume. An additional advantage of using greater than or equal to two sanitization reactors in parallel is the added flexibility, for example that the anaerobic process can be optimized, and run independently from the sanitization, where partial volumes of the contents of the biogas reactor are transferred to at least one sanitization reactor. The heat that develops in the initial biological treatment process is used to amplify the effect of the ammonia sanitization step; and heat recovered from other unit operation, or parts of the process can be used, such as residual heat in the sanitized end product, or surplus heat from generators, boilers, pumps etc. Consequently, the ammonia sanitization step is performed at lower pH than required, with less addition of alkaline chemicals, using shorter hold-times, in a smaller sanitization reactor, without compromising the result. The method and system can be implemented in existing biogas production plants as well as in the design of new plants. Compared for example to alkaline sanitization alone, the combined biological treatment and ammonia sanitization can be performed at a lower pH, requires less time, smaller addition of alkaline chemicals, hence affords larger throughput. Similarly, compared to biological treatment alone, the combined biological treatment and sanitization requires shorter hold-times, and affords larger throughput. The method and system is also suitable for scale-up. Effective and reliable sanitization of the digestate offers extra security, and makes it possible to use different waste fractions for the production of biogas. The digestate can then be used as fertilizer also on land intended for edible crops, providing for an improved recirculation of nutrients and reducing the need for chemical fertilizers. When nitrogen containing material, e.g. urea, is used as the alkaline material, it affords the additional advantage of increasing the nutritional value of the sanitized end product. The method and system allows use of effluent streams having low dry matter concentration. The initial biological treatment step, the production of biogas, does not have to be taken to the high temperatures normally required for sanitization by temperature alone, which reduces the hold-time and increases throughput. The relatively low investment cost compared to installing municipal sewers in areas where this is not present, e.g. rural and sparsely inhabited areas. Thus, the method and system is an alternative to expanding the municipal sewer network, and makes it possible to develop areas that are difficult to access, and where the sewage handling has been limiting factor for expansion and development.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) system for production of biogas under anaerobic conditions, comprising biogas reactor (30) and at least one sanitization reactor (40), adapted to adjusting pH of the digestate to at least pH 7, preferably to about 9, especially above 9, and for incubating the digestate at, at least 30 degrees C; and    (2) sanitization reactor adapted for use in the system.    DESCRIPTION OF DRAWING(S) - The figure shows schematic of system for production of biogas.    Receiving tank (10, 20)    Biogas reactor (30)    Device for feeding starter culture (32)    Sanitization reactor (40)    Mixer (31, 41)    Feeding (42)    Storage tank (50, 60)    Device for temporary storage (90)