Sustainable services

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During the treatment of urban waste water with mechanical-biological processes, solid substances defined as sewage...

During the treatment of urban waste water with mechanical-biological processes, solid substances defined as sewage sludge are being separated, at various stages. The sewage sludge can be considered the residue of the waste water treatment plant, namely the unwanted product of its operation, along with the intended product, which is treated water. The sludge is processed in the treatment plant in order to be stabilised and reduced in volume. The sludge processed to a form usable for spreading on soil is defined as BIOSOLID. The diagram presents the stages of formation, separation, processing and final disposal of sewage sludge.



solid or partially liquid material produced and separated by treatment processes applied to drinking water or waste water



complex mixture of organic residues with content of nutrients, nitrogen and phosphor, soil particles, sediments and micro-organisms and water, which complies with the legal requirements for recycling usage

Biosolid is being used for regenerating soil, in particular in degraded areas, for improving the soil’s nutrient contents, mainly phosphor, which usually is in deficit. The biosolid improves the microbial activity in the soil and the cycle of nutrients needed for vegetation growth. The usage may be extended to forested areas, to degraded areas such as surface mining areas, landfills and arid zones poor in humic components. Application of biosolid on soil is performed taking into account the composition, structure, crops and climate conditions.

Success of agricultural usage, for soil regeneration or to support vegetation growth is conditioned by accurate application of legal requirements and by competent and detailed monitoring.

Final disposal of sludge and transformation into bio-soil comprises political, economical and technical aspects.

Any solution, including the application on soil, has its pros and cons. The decision, once made, will be verified exclusively over time; risk assessment is required, but should not stop the stakeholders from making a decision. The success of implementing the decision to recycle by soil application largely depends on the monitoring and control procedures, including the sampling and analysis procedures.

The aspects mentioned above lead to the conclusion that the application of biosolid on soil represents the best ecological option. This option will be assessed based on the rule of 2000 kg nitrogen/hectare. To be reminded that fertile soil is characterized by values of 5000 – 20000 kg nitrogen (N)/ha.


Q: toxic or polluting elements accumulate in soil, with an effect of “time bomb”?

A: practice proves that heavy metals are immobilised in soil for a pH above 6.5. Organic compounds (OC) are decomposed in the natural environment of the soil. 

Q: the smell of biosolid can affect human health?

A: the smell caused by compounds of sulphur and nitrogen quickly dissipate after application on soil. The toxicity of such gases/volatile compounds is present only for high concentrations. Smelling when stored is caused by improper practices. Since the use of unstabilised product is not allowed, the aspect of smell may be solved with proper practices. 

Q: pathogen agents can contaminate air and water, causing diseases?

A: regulations allow for the application of sludge with 99% reduction of pathogens, furthermore the intestinal pathogens would not resist to soil’s natural environment, therefore the proper sanitary conditions are secured. To not forget that application of raw sludge or injection of liquid sludge are forbidden. 

Q: pollutants or salts in the biosolid contaminate natural water bodies?

A: compliance with provisions regarding the amounts and frequency of application, which are extremely conservative, will prevent the contamination of surface or ground water. However, synthetic chemical fertilisers caused, by diffuse pollution, extremely severe cases of contamination, in particular in ground water. Also, the biosolid has a low content of salts, which are mostly retained in the treated water. Application on soil is the natural way of eliminating and only way of recycling the sludge processed into biosolid, and includes the principle “nature knows best”. Also to keep in mind another ecological principle, namely “there is no free lunch”.

  • Recycling and reintegration into nature
  • Enrichment of soil with nitrogen and phosphor
  • Positive effects on agricultural productivity
  • Economic option, little investment compared to other products
  • Requires labor
  • Limitations and seasonal restrictions
  • Storage capacity
  • Dependence on climate and weather conditions
  • Population’s reserve / opposition

Any negative impact on the environment is a consequence of noncompliance with legal provisions or application, control and monitoring procedures.

RESTRICTIONS for farmers intending to use biosolid in agriculture:

The farmers should:

  • use only treated sludge, which received application approval from the local environment protection authority, based on the special agrochemical study performed by the Office of Pedology and Agrochemical Studies (OPAS/OSPA) and approved by the Department of agriculture and rural development. At farmer’s request, the sludge supplier has to present a copy of the application permit.
  • not apply sewage sludge on pastures, in orchards or vegetable production land in less than 10 months prior to harvest or during harvest period.
    not apply sewage sludge on land with gradient above 12%.
  • apply sludge only when access to land is possible, and to incorporate the sludge into soil immediately after application.
  • notify competent authorities and sludge supplier about crop rotation.
  • notify the sludge supplier if cancelling the application of sludge prior to shipping.

Additional information regarding the obligations of sewage sludge suppliers and users, usage amounts and restrictions, spreading equipment and season, other particular aspects of usage etc, according to national regulations in force (Order no. 344/2004 approving the technical norms regarding the protection of environment and in particular of soil when using sewage sludge in agriculture) can be found at on the Ministry of Environment and Forests website.


biosolid – recycling product, protects and regenerates soil, reduces phosphor consumption, which is in deficit;
nutrients – nitrogen and phosphor, in insoluble form, as opposed to synthetic fertilisers, providing a slow assimilation and reducing the contamination of ground water;
controlling and monitoring the application, introducing a local management based on general regulations, reporting and recording “complicate” the farmers’ life;
use of biosolid reduces the need for pesticides.

The use of sludge in agriculture has the advantage of replacing the conventional fertilisers with sludge fertiliser, improving the physical and organic properties of soil by adding stabilised organic substances.

Users should be aware that only treated sludge which received application approval from the local environment protection authority, based on an approved special agrochemical study, can be used in agriculture.

Sludge usage should be adapted to local conditions and take into account the environment conditions, existing pollution and specific purpose of particular land areas.

The sludge supplier (Regional Operator) is responsible for everything regarding the quality, quantity, transport and spreading of sludge on agricultural areas, as well as for its effect on the environment and human health after application.