UV light-activated highly efficient photoelectrocatalytic decomposition of biorelated compounds and biowastes was successfully achieved by a Biophotochemical Cell (BPCC) comprising a nanoporous semiconductor film photoanode and an O-2-reducing cathode with high internal quantum efficiency over 100. Major decomposition reaction was dark auto-oxidation of the activated substrate radicals (R-aEuro cent) by the bulk O-2 into CO2 or/and to N-2. Photocatalytic decomposition of aqueous solutions of urea, ammonia, pig urine, and oxalic acid was investigated. Combination cells were fabricated comprising a photoanode/cathode unit for activation of organic compounds and a large volume chamber for autooxidation of activated substrates.