NOVELTY - Culture medium comprises biogas digester waste liquid, sodium silicate, potassium dihydrogen phosphate, ammonium nitrate, mannitol, auxin solution, sodium bicarbonate, sodium malate, ferric citrate, trace element solution, indoleacetic acid, sodium acetate, sodium glutamate, sodium benzoate, magnesium stearate, sodium carboxymethyl cellulose, D-sodium erythorbate, propylene glycol alginate, beta -carotene, alpha -naphthyl acetate, soil leaching solution, mud leaching solution and seawater (sterilized with chlorine dioxide).    USE - The culture medium is useful for culturing N.directa (claimed).    ADVANTAGE - The culture medium is environmentally-friendly and cost-effective, provides nutrients to culture, and increases production rate by 340%.    DETAILED DESCRIPTION - Culture medium comprises 85 ml biogas digester waste liquid, 5 mg sodium silicate, 8 mg potassium dihydrogen phosphate, 6 mg ammonium nitrate, 3 mg mannitol, 2 ml auxin solution, 200 mg sodium bicarbonate, 2 mg sodium malate, 5 mg ferric citrate, 13 ml trace element solution, 1 mg indoleacetic acid, 5 mg sodium acetate, 10 mg sodium glutamate, 5 mg sodium benzoate, 8 mg magnesium stearate, 12 mg sodium carboxymethyl cellulose, 3 mg D-sodium erythorbate, 5 mg propylene glycol alginate, 12 mg beta -carotene, 0.2 mg alpha -naphthyl acetate, 1 ml soil leaching solution, 1.2 ml mud leaching solution and 1000 ml seawater (sterilized with chlorine dioxide). The auxin solution comprises 1 mg of biotin, 100 mg vitamin B1, 0.1 mg nicotinic acid, 10 mg p-aminobenzoic acid and 1000 ml distilled water. The trace element solution comprises 1000 mg EDTA, 1 mg cobalt chloride, 5 mg zinc chloride, 40 mg copper chloride, 40 mg manganese chloride, 10 mg ferric chloride and 100 ml distilled water. An INDEPENDENT CLAIM is included for method for culturing Navicula directa using the biogas digester waste liquid as raw material, which involves obtaining 25 l culture vessel made from food-grade polyethylene material having good transparency to facilitate photosynthesis of N.directa, sterilizing culture vessel using hydrochloric acid for 10-15 minutes, washing the culture vessel using cold seawater for 3 times, inoculating 6x 104 cells/ml N.directa into culture vessel, supplying mixed gas comprising air, pure carbon dioxide, ozone and methane in a weight ratio of 92%:6%:1%:1%, culturing medium at optimum temperature of 25 plus minus 2 degrees C, and optimum light intensity of 4000-9000 Lux, placing culture medium under energy-saving lamps, maintaining intensity at 2000-4000 Lux, culturing medium for 8 days, adding sterilizing liquid to medium, and sterilizing medium for 30 minutes, where the sterilizing liquid is prepared by adding 85% ethanol to medicines and 3-5 pts. wt. auxiliary materials, where the medicines comprises 30-50 pts. wt. Pseudolarix, 15-25 pts. wt. Polygonum perfoliatum, 15-25 pts. wt. Sophora, and 5-15 pts. wt. Artemisia argyi.