Number

312-EN

Section

General Section

Use

Abstract

The alternative stabilises the secondary activated sludge from waste water treatment facilities using only physical processes. It makes unnecessary the polymers and strong acid or alkali substances that are generally used to treat this type of sludge and it reduces considerably costs, energy and land use associated with conventional waste treatment.

Substituted substances

Reliability of information

Internet information: data are from an internet document and only a basic and partial evaluation could be performed

Reason substitution

skin/respiratory sensitizing
CMR

Other type of alternative

Technology with various physical processes run concomitantly or sequentially

Hazard Assessment

Substances to be substituted: Sulphuric acid causes severe skin burns and eye damage. It has strong inorganic acid mists which are category 1 carcinogen (IARC) as listed in the Substance Database according to SUBSPORTplus Screening Criteria (SDSC). Sodium hydroxide is not listed in the SUBSPORTplus Database. Polyacrylamide is not listed in the SUBSPORTplus Database and has no harmonised classification according to Annex VI of Regulation (EC) No 1272/2008 (CLP Regulation). Alternative substance: It is a physical and biological process that cannot be conclusively evaluated at this point in terms of hazards.

Description of Substitution

Biological treatment of waste water is applicable after a previous first stage has removed part of the solids as primary sludge. In the biological stage bacteria are used for treatment, because they feed on organic bio-degradable pollutants dissolved in water. Their cells can then be easier removed than dissolved organics by different methods that separate them as secondary sludge. Part of this sludge is redirected in the biological treatment stage to seed it. The rest is disposed of as waste, in order to maintain an appropriate nutrient/ microorganisms ratio and to avoid surpassing the capacity of the treatment facility. Before disposal or recycling the secondary sludge has to be treated in order to stabilise it, to reduce its water content and its volume. Without treatment the sludge cannot be sent to landfill. This is a costly process, usually involving chemical and physical means. Acids or alkali are used to set pH at very low/high values (pH2 or 12) in order to interrupt biological processes that might continue after the final disposal. Polymers are used in the dewatering and in the drying stages. Moreover if nutrients are not enough, they need to be added in the biological stage since they as well as oxygen are essential for the treatment. An alternative technology for secondary waste activate sludge (WAS) treatment uses only physical processes to stabilise WAS. The treatment includes a combination of physical processes that are run simultaneously or successively within multiple reactors. The intensity of these processes has a high impact on cells, including cell nuclei. Explosive decompression, cavitation, micro-bubble oxidation are achieved in a concentration chamber using pump sand Venturi tubes, followed by a decompression tank. Destroyed micro-organisms can no longer develop but they provide useful nutrients for the biological treatment when the waste sludge is recycled in the bio-reactor. Moreover the high pressure oxidation eliminates odours while supersaturated oxygen flow supports aerobic degradation. Non- biodegradable materials and salts are also recycled to the bioreactor. When they build up to a level that is disturbing for the process, the installation needs to have a purge. The frequency of purging depends on the sludge type and the capacity of the facility and it can be between several weeks and several months. According to the producer polymers typically used as sludge thickening agents and those used to assist in the drying of secondary sludge are not necessary. It also minimises the use of odour masking chemicals and of additive nutrients. The chemicals used for sludge stabilization can also be eliminated. The alternative reduces costs with transporting and handling of chemicals, reduces corrosion of facilities, gaseous emissions and odours and pollution due to transport and landfill leachate. It has a small footprint area (about 26 square meters) and a height of less than 3 meters. The facility can treat about 1000 cubic meters of sludge per day.

Case/substitution evaluation

The alternative needs changes of the treatment facilities. Initial investment costs are balanced by the reduction in energy and chemical costs. The important reduction in the finally disposed waste also cuts costs. By employing physical processes the alternative eliminates the use of chemicals like sulfuric acid or sodium hydroxide which are corrosive and of polymers, like the copolymers of polyacrylamide , which are not hazardous by themselves but may decompose into acrylamide, that may cause cancer , damage reproductive functions and act as neurotoxicant. Unstabilised waste generates hazardous leachate that may infiltrate into the ground water polluting it and generates greenhouse gases like methane.

State of implementation

In use

Type of information supplier

Producer / distributor

Type of publication and availability

Internet presentation

Publication source: author, company, institute, year

The presentation is based on the information published by the producer Oxycair Technologies, Canada on its website.

Date, reviewed

November 26, 2021