HSAD: Anaerobic Compost

HSAD recycles organic waste into fuel gas and a nitrogen-rich product that can be filtered to yield liquid fertilizer and anaerobic compost.

Anaerobic compost is a humus-rich soil amendment made in the absence of oxygen. It improves soil and plant health, prevents erosion and holds moisture and nutrients in the soil. It also reduces the need for fertilizer, improves soil drainage and provides micro-nutrients. Anaerobic compost can be used in projects ranging from agricultural lands requiring a high quality product to commercial land, industrial land and land reclamation.

Known as "oufei" in China, anaerobic compost was in use there before the Song Dynasty (960 AD) and has sustained rice yields averaging 4 tons per hectare for centuries. This occurred because anaerobic composting reduces organic matter to ammonium, which is the ideal fertilizer for paddy rice and many other crops.

Most compost available today is "aerobic", or made in the presence of oxygen. The aerobic process can take three weeks to two years and requires a fair amount of mixing and human supervision. In contrast, making anaerobic compost with the HSAD technology takes one to three weeks and requires little attention because it is almost completely automated. In addition, the HSAD process produces energy.

The composition of HSAD compost and liquid fertilizer depends on the waste treated. The following tables contain data for anaerobic compost obtained from a mixture of municipal solid waste and food processing sludge treated in an HSAD facility in Stanton, California.

Table 1. Anaerobic Compost from HSAD: General Parameters
Parameter Presence in Anaerobic Compost
Moisture 63.43 %
pH 8.2
C:N Ratio 22:1
Soluble Salts 25.6 dS/m
Organic Matter 57.92 %
Ammonia Nitrogen <0.005 %
Nitrate Nitrogen <0.005 %
Chloride 2.56 %
Organic Nitrogen 1.54 %
Volatile Solids 57.92 %

Table 2. Anaerobic Compost from HSAD: Organic Amendment Report (on dry basis)
Metal Presence in Anaerobic Compost
N - Nitrogen 1.54 %
P - Phosphorous 0.75 %
P2O5 - Phosphate 1.72 %
K - Potassium 1.78 %
K2O - Potash 2.14 %
S - Sulfur 0.69 %
Mg - Magnesium 1.18 %
Ca - Calcium 3.33 %
Na - Sodium 1.40 %
Fe - Iron 5370 ppm
Al - Aluminum 5580 ppm
Mn - Manganese 570 ppm
Cu - Copper 72 ppm
Zn - Zinc 440 ppm
B - Boron 60 ppm

Table 3. Anaerobic Compost from HSAD: Heavy Metal Concentrations (on dry basis)
Metal Presence in Anaerobic Compost
Arsenic 3.1 mg/kg
Cadmium 0.2 mg/kg
Chromium 36.9 mg/kg
Cobalt Not Available
Lead Not Detected
Mercury 0.06 mg/kg
Molybdenum 6.9 mg/kg
Nickel 10.4 mg/kg
Selenium Not Detected
Zinc 444.2 mg/kg

The HSAD compost and liquid fertilizer meet the most stringent standards regarding maturity, organic matter content, absence of physical contaminants, pH, particle size, nutrient content, moisture content and trace element content. They also meet "Class A" pathogen requirements from the U.S. Federal Standards (40 CFR Part 503) that regulate the beneficial use of biosolids.