Disposal of wastewater sludge (WWS) without sludge beds by processing into marketable products

Current status of WWS disposal

Main directions of WWS disposal:

1. Burial on sludge beds.

2.  WWS recycling as fertilizer.

3. Direct combustion method.

4. Pyrolysis recycling method .

1. Deposition on the sludge beds.

        At present sludge beds process 90% of the total sludge produced in Russia. Evaporation from silt detention ponds stands for air pollution and soil filtering stands for pollution of ground waters and nearby water bodies. Harmful gases generated by WWS exceed the maximum allowable concentration and smell bad. Their smell is 4 - 5 scores on the organoleptic indicator scale  

2. WWS recycling as fertilizer.

 Numerous studies have confirmed the possibility of disposal of sediments of urban wastewater as organic fertilizers. The sediments relate to Hazard Class IV (low-hazard waste).

        WWS recycling is limited by two factors: sanitary-epidemiological and sanitary chemical. Modern technologies eliminate the epidemiological risk.

However, toxic components contained in sediments (Cd, Cu, Ni, Pb, Cr, Zn, Hg, As, Mn, and in some cases, Mo, Se, Co, Sr, B, Be, Ba) that exceed background levels in the natural objects obstruct their recycling in agriculture and impede the use of such sludge as ‘recultivants’ of disturbed lands. To prevent environmental pollution by heavy metals (HM) the majority of countries have established national standards for maximum permissible content of heavy metals and WWS to be recycled.

       Nevertheless, in Western Europe many farms which grow environmentally-friendly fertilizer products  reject them.

3. Disposal by direct combustion.

The WWS direct combustion method includes combustion in a boiling (pseudoliquified) layer implemented by BAMAG GmbH (a company from Germany) in St. Petersburg.

The boiling layer technology enables to burn the material in a flare of hot sand raised by an air stream; slag with harmful oxides which are to be subjected to a multistage treatment goes away with the exit gases. 

Such plants were massively built in U.S., Japan and some European countries in the 80s of the XX century. However, relation to the sludge combustion technology became rather negative in the 90s due to its negative impact on environment. 

It shall be noted that in recent years many industrial waste combustion plants (including wastewater treatment combustion plants) have been closed in Europe and Asia.

European countries are gradually withdrawing from WWS direct combustion method and switch to WWS treatment methods which enable to get marketable products and at the same time minimize operating costs.

4 . Recycling by pyrolysis.

      The most progressive method. During pyrolysis (oxygen-free thermal decomposition of organic substances) at a temperature not higher than 700 °C, one obtains a combustible gas ( ~ 55 %) , char (~ 35 %) , liquid organic materials ( ~ 15 %), which fly away with exit gases at these temperatures and char is subjected to semi- gasification process and also turns into a combustible gas.

      The metal oxides remain in the gasification chamber in the form of pure slag suitable for use as a mineral filler.

       WWS only I subject to gasification and pyrolysis of organic constituents , so emissions do not contain harmful substances , as in the direct combustion method. Table 1 shows the composition of WWS-based combustible gas indicating the absence of harmful components.

Pure slag

Char

Dry remains

Таблица 1