We have discussed what Class A biosolids is and even how safe it is to be used as fertilizer for food production and other agricultural functions. Like Class A biosolids, Class B biosolids also contain important nutrients and organic matter, making them viable as fertilizers for certain types of plants and vegetation.
Class B biosolids goes through the same process as Class A biosolids, but they are allowed to have higher levels of detectable pathogens. Though they are also treated, they require a permit from the US EPA for land application, crop harvesting, and public access.
The US EPA maintains that Class B biosolids is generally safe, but the safety requirements it has set for this particular category is insufficient to ensure safety. There have been cases wherein forage crops grown in soil fertilized with Class B biosolids have died. Humans working on fields where Class B biosolids was applied fell sick or ill. The level of contaminants that this class contains is enough for the public to be wary about consuming any food grown in Class B biosolids fertilizer.
Restrictions on metal contaminants
For Class B biosolids, the US EPA has released a guideline on the restricted metal contaminants in sewage sludge. The limits for concentrations are listed below in dry-weight value.
Arsenic, 75ppm / Cadmium, 85ppm / Chromium, 3000ppm / Copper, 4,300ppm / Lead 840
ppm / Mercury, 57ppm / Molybdenum, 75ppm / Nickel, 420ppm / Selenium, 100ppm / Zinc, 7,500ppm.
Other restrictions for Class B biosolids include not being viable for use in home gardens and home lawns.
In terms of food production and consumption, here are the restrictions:
- The edible parts of food crops, feed crops, and fiber crops must not touch the surface of the soil and they shall be harvested only after 30 days of the application of biosolids.
- Those with parts that touch the biosolids or soil mixture and are totally above ground should not be harvested until 14 months after application.
- Those with parts below the land surface where the biosolids remain for four months or longer shall not be harvested until 20 months after the application.
- Those with harvested parts below the land surface where biosolids remain for less than four months shall not be harvested until 38 months after application.
- Turf grown on land where biosolids are applied shall not be harvested until one year after application of the biosolids. This is applicable when the harvested turf is placed on either land with a high potential for public exposure or a lawn.