Biosolids from Sludge: What Are They and How Are They Used?
Describes the nature and beneficial use of biosolids (treated domestic sewage sludge), water treatment plant residuals, and domestic septage in Texas; and how biosolids are classified.
Biosolids
During domestic wastewater treatment, the liquids are separated from the solids. Those solids are then treated physically and chemically to produce a nutrient-rich, primarily organic product known as sewage sludge. Domestic wastewater treatment plants (WWTPs) must dispose of the sewage sludge generated by the treatment process. They may do so by sending it to a TCEQ-authorized land application site, landfill, other WWTP, or a facility that further processes sewage sludge.
Biosolids–Sewage sludge that has been treated to meet certain pathogen standards before beneficial use. Biosolids also may be disposed of by incineration, landfilling, or other forms of surface disposal.
Beneficial Use
Beneficial use via land application is the spraying or spreading of biosolids, water treatment residuals, or domestic septage onto agricultural land at a rate that does not exceed:
- The amount of nitrogen needed to support crop growth and soil productivity.
- The metal or toxic constituent limitations.
Benefits of Land Application
- Improves soil properties–such as texture and water holding capacity–which make conditions more favorable for root growth and increases the drought tolerance of vegetation.
- Supplies nutrients and micronutrients essential for plant growth.
- Can also serve as an alternative or substitute for chemical fertilizers.
To be used in this way, biosolids must meet state requirements to process, handle, and land-apply them in a manner that minimizes potential risk to human health per Title 30, Texas Administrative Code (30 TAC), Chapter 312 and the federal rules in Title 40, Code of Federal Regulations (40 CFR), Part 503.
Classes of Biosolids
In Texas, biosolids are divided into three class designations–Class A, AB, and B–based on treatment methods. The different classes have specific treatment requirements and general requirements for management practices. There are treatment requirements for:
- pollutants (heavy metals)
- pathogens–like disease-causing bacteria and viruses
- vector attraction reduction–like odor control
Class A
Class A biosolids–including lime stabilized biosolids and composted biosolids–are treated to reduce pathogens and vector attraction. In addition to pathogen treatment, they must do one of the following:
- Reduce the density of fecal coliform–a type of bacteria–to less than 1,000 most probable number (MPN) per gram of total solids.
- Reduce the Salmonella density to less than three MPN per four grams of total solids.
If biosolids are sold or given away in bags or bulk for lawn or home garden application, the biosolids must meet Class A requirements.
Class AB
Class AB biosolids–a class unique to Texas–achieve the same bacteria levels as Class A but through different treatment processes. They include additional buffer zone and odor control requirements.
A site permit is not required for a person or third-party applying Class A or AB biosolids.
Class B
Class B biosolids–less stringent than A or AB–must also reduce bacteria, but a higher concentration is allowed: 2,000,000 MPN or colony-forming units of fecal coliform per gram of total solids.
A permit is required to land apply Class B biosolids. The permit holder must adhere to all applicable site restrictions and management practices in 30 TAC Chapter 312.
Restrictions include when biosolids may be land applied before:
- harvesting crops or turf
- animal grazing
- public contact with the site
Home use of Class B biosolids is prohibited. Biosolids that do not meet Class B requirements cannot be land applied.
Other Wastes That May Be Land Applied for Beneficial Use
Other types of waste that TCEQ authorizes for beneficial use by land application include domestic septage and water treatment residuals.
Domestic Septage
Domestic septage can contain a variety of substances depending on the type of waste being treated in the septic system, usually:
- Water, sewage, inorganic materials like grit, and organic fecal matter.
- Small amounts of substances from normal household activity like cleaning supplies, pharmaceuticals, etc.
Not classified as domestic septage:
- Grease trap wastes and chemical toilet wastes.
- Commercial and industrial septage.
Any mixture of domestic and industrial septage causes the entire batch to be considered non-domestic septage. Only domestic septage is eligible for land application under 30 TAC Chapter 312.
Water Treatment Residuals
This type of waste is generated during surface water or groundwater treatment for potable use. These residuals include sediment from the source water that is filtered or settled out as part of the water treatment process. The residuals from water treatment plants:
- Contain some nutrients–but to a lesser degree than biosolids or domestic septage–and trace minerals and elements.
- Do not include pathogen and odor concerns.
Contact Information
Transporting sewage sludge or biosolids, water treatment residuals, or domestic septage:
Sludge Transporter Registration Program
P.O. Box 13087, MC-129
Austin, Texas 78711-3087
512-239-6413
streg@tceq.texas.gov
Domestic septic systems (on-site sewage facilities):
On-site Sewage Facility Program
P.O. Box 13087, MC-235
Austin, Texas 78711-3087
512-239-6390
ossf@tceq.texas.gov
Biosolids or land application for beneficial use:
Land Application Program
P.O. Box 13087, MC-150
Austin, Texas 78711-3087
512-239-4671
biosolids@tceq.texas.gov