In 2008, Joint Stock Company "Apa-Canal Chisinau" together with experts from Netherlands and Poland started the implementation of municipal sewage sludge processing using the geotubes method - bags for municipal sewage sludge dehydration as a solution for bad odour removal. Sewage sludge dehydration process using the „geotubes” method takes place rapidly, in about 7-20 days. This process is catalyzed by reagents that ensure the clogging of municipal sewage sludge.
From September until the end of 2009, a pilot Project on municipal sewage sludge dehydration in geotubes started and it was conducted by "Apa-Canal Chisinau" in cooperation with the Dutch company „TenCate”; then, the activities planned under the Project Execution “Geotubes” were developed. Within the pilot Project there were dehydrated about 90 000 m3 of raw sewage sludge. In this regard, there were used 40 bags of different size, occupying only 1.25 ha of platforms, while the use of previous technology would take 6 ha. Also, the municipal sewage sludge that was pumped in this period was dehydrated within one month. Previously, it took about 18 months.
Dehydrated sewage sludge in geotube bags is discharged at the storage place once a year in the cold period – in the period from December to February, when the release of specific odours and infections is minimal. The Wastewater Treatment Plants from Europe activate quite similar to this (C. Becciev, 2011). The difference is that European technology of “geotubes” is not used for as large enterprises as in Chisinau. According to data presented by the Joint Stock Company “Apa-Canal Chisinau”, in the past two years, about 110-115 thousand m3 of dehydrated municipal sewage sludge, with the humidity of 78-80%, using the “geotubes” method is accumulated annually in the Wastewater Treatment Plant. In connection with the production of this new form of sewage sludge it was necessary to study its composition. The research was performed on dehydrated sludge which was transported to the fermentation landfill in December 2010. The samples included the municipal sewage sludge pumped to the Wastewater Treatment Plant in the period October 2011 - October 2012.
Compared with the traditional technology (open air drying beds), the period of dehydration is shorter; carbon content (21.8% of dry mass) and total nitrogen content (2.6%) were approximately two times higher, total phosphorus content (2.8% P2O5) recorded an insignificant decrease (10%) while potassium remained at about the same level. The studied municipal sewage sludge is characterized by a weak alkaline reaction. The pH value ranged from 7.1 to 7.8, forming the average of 7.4 ± 1.1. The humidity was on average of 65.1 ± 26.2%. The content of organic matter of the sewage sludge in natural humidity was of 15.1 ± 6.7%.
Chemical composition demonstrates that municipal sewage sludge dehydration using geotubes represents an important source of organic matter for soil and nutrients for agricultural crops. Sewage sludge is rich in total nitrogen, especially in phosphorus – element that lacks in about 76% of Moldova’s agricultural soils. Mobile forms of nitrogen and phosphorus make up about 14-17% of the total content. One of the most important factors limiting the use of municipal sewage sludge as organic fertilizer is the excessive content of heavy metals. The main source of heavy metal content is the wastewater from industrial enterprises, but due to changes that occurred in the structure and volume of production issued by the industrial enterprises from Chisinau municipality, there were recorded significantly decreased volumes of wastewater. As to data obtained on the content of heavy metals and comparing them with the limiting values, we found that they do not exceed the allowable concentrations developed by the industrialized countries.