Highly polluted wastewater accumulates in milk collection stations. Its biochemical oxygen demand (BOD5) ranges from 369 to 1388 mg O2 lE-1. The average contamination was 600 mg O2 lE-1. There was a comparatively small amount of biogenic materials in it: total nitrogen – 27.3, total phosphorus – 5.0 mg lE-1. When variations of wastewater amounts are high, sand filters of vertical filtration are successfully used for the treatment of household wastewater. Model investigations were carried out to establish the possibilities of using these filters in treating wastewater in milk collection stations. The efficiency of wastewater treatment with these filters was 99.3% by BOD5, 99.1% – by suspended solids, 91.2% – by total nitrogen and 98.8% – by total phosphorus. According to the research results dependency equations were made; they were used to calculate the amount of main contaminants in milk collection stations that infiltrated through a layer of silt of diff rent thickness. The calculations showed that wastewater treatment to permissible limits is ensured by filtration through 0.6 m thick layer of sand. The average treatment level by BOD5 in such a filter of vertical filtration would be 20 mg O2 lE-1 and the treatment efficiency – 95.9%. Treatment efficiency of total nitrogen would be 88.1%, total phosphorus – 96.9, suspended solids – 95.5%. Preliminary research in the model showed that sand filters of vertical filtration can be used to treat wastewater from milk collection stations.

The objective of the present studies is to estimate a distribution of nitrogen and phosphorus compounds in filter media of horizontal filters. The studies were carried out in 2009 within two wastewater treatment facilities of horizontal flow in Lithuania. From both treatment facilities sand samples of different depth and separate profiles in the direction of water flow were taken. These samples were used to determine sandmoisture and concentration of N-NO3, N-NH4 and total P. The measurements of sand moisture showed that the filtration of wastewater through the sand is in process only in the lower layer of sand filter and near the distribution pipe. Moisture regime in the sand influences the composition of nitrogenous compounds in it. In the part of the filter, where sand's moisture is lowest, nitrate concentration is highest. Ammonia concentrations are completely opposite. In the lower part of the filter anaerobic conditions are dominant and nitrification processes stop. Analysis of phosphorus concentrations in filter's sand showed similar tendencies as nitrogen analysis. In the upper part of the sand phosphorus concentration turns out to be 2-3 times lower than in the lower part of the sand. In the filters of horizontal flow, part of the sand contributes little or does not contribute to the wastewater treatment process at all; therefore, it is advisable to use wastewater recirculation for the improvement of nitrogen and phosphorus removal. For that purpose in the part of the filter not included in the treatment process the filter of vertical flow could be arranged.

Modelling of the domestic wastewater treatment and removal of the Ptotal in the vertical filters with the sand, dolomite powder and dolomite chippings each separately used as a filter media was performed. Six filter models taking 0.2 m2 each were installed. The length of the filtration path was equal to 0.8 m in three of these models and 0.4 m in the other three models. The hydraulic load on the filters was equal to 0.03 m3 mE-2 dE-1 in 2008, and it was equal to 0.06 m3 mE-2 dE-1 in 2009. The domestic wastewater which had undergone the pre-cleaning in the septic systems was poured into the models. Their average pollution with the Ptotal reached 14.6 mg LE-1 in the first year and 9.2 mg LE-1 in the second year. When the filtration path was equal to 0.8 m and the hydraulic load was equal to 0.03 m3 mE-2 dE-1, the Ptotal was removed from the wastewater with the efficiency of 99.9% in the dolomite powder filter. In the filter models with the sand and dolomite chippings, the Ptotal was removed with the same efficiency of 87.7%. When the hydraulic load doubled, the efficiency of P removal decreased to 59.8% in the sand filter and down to 45.7% in the dolomite chippings filter. When the hydraulic load reached 0.06 m3 mE-2 dE-1, only the dolomite powder filter could ensure the adequate cleaning of the domestic wastewater up to the allowable norms.

The article explores the possibility of the vertical filter instead of sand used for more coarse media for recharging, which would reduce the risk of freezing and clogging and ensure the cleaning of wastewater up to the limits. For this purpose the dolomite chippings have been chosen for research. It was equipped with two 0.2 m2 filters area: one of their models was 0.4 m dolomite chippings layer, the other 0.8 m. They have been loaded by the wastewater after the primary cleaning in the septic system. Medium contamination in organic pollutants have been 358 mg O2 LE-1, Ptotal 14.6 mg LE-1, Ntotal 108.5 mg LE-1, suspended solids 161.1 mg LE-1, pH 7.8. Hydraulic load of the filters was 0.03 m3 mE-2 dE-1. Wastewater flowing through a vertical sand layer by an average of 98.2% of the organic pollutants to be detained. The average of effluent wastewater contamination of the filter was 22 mg O2 LE-1. In the filter with dolomite chippings media can be cleaned wastewater with pollution reaches 500 mg O2 LE-1. The maximum load of organic pollutants is 14 mg O2 LE-1. The filtration path length to ensure wastewater cleaning up tolerance 0.8 m. In the filter with dolomite chippings media Ptotal is cleaned even 87.0%, Ntotal only 13.0% efficiency.

In 2003 an experimental model of the dairy waste water cleaning facility was equipped for cleaning the wastewater of the milk collection station. We examined the possibilities to destruct the organic pollutants present in the dairy wastewater in the sand filters of vertical and horizontal filtration. The pollution level of the dairy wastewater is from 2 to 4 times higher than that of the domestic wastewater. The dairy wastewater was cleaned following the procedure: cleaning in the aeration-flocculation system-cesspool-two stage sand-reed filter. The pollution of the wastewater leaving the cleaning facility never exceeded the allowable norms. The average BOD5 was 4 mg O2 LE-1 and the maximal value was 9.6 mg O2 LE-1. The investigation revealed that the horizontal sand-reed filter with the filtration path of 7 meters ensured cleaning of the dairy wastewater up to the allowable norms. The organic pollutants were cleaned with the efficiency of 97.2% after filtration through such a sand layer. In order to find out the possibilities of vertical filtration filters use for cleaning the dairy wastewater leaving the milk collection stations, we performed a modelled investigation. The wastewater cleaning efficiency in respect to BOD5 reaches 99.3% in these filters. The calculation according to the regression equation received showed that filtration through 0.6 m sand layer ensured successful cleaning of the wastewater up to the allowable norms. In such a vertical filtration filter the average BOD5 cleaning level was 20 mg O2 LE-1.

The objective of the studies was to evaluate the suitability of sand reed filter for the treatment of wastewater from milk collection post and on the basis of the studies performed to determine the minimal length of filtration path in order to reach the normative wastewater treatment level. In 2003, in Padargupiai village (Ariogala municipality, Raseiniai district) experimental treatment facilities for wastewater from milk collection post were constructed. As the pollution of such wastewater after pre-treatment (primary treatment) is 2 times higher than that of domestic wastewater, the filtration path in constructed wetlands was made 2 times longer (up to 10 m long). On the basis of the study results the dependence equations were made, with the help of which the amounts of the main pollutants contained in wastewater of milk collection posts were calculated, after the wastewater is filtered through the sand layer of different length. As calculations have shown, the treatment of wastewater from milk collection posts until the allowable rates depends on the constructed wetlands with the length of the filtration path of 7 m. The removal of organic pollutants, total nitrogen and total phosphorus from wastewater flowing through such media are 97.2%, 68.7%, and 91% respectively. The studies performed allowed to improve the calculations of the necessary area of constructed wetlands in order to reach the normative treatment level of wastewater from milk collection posts according to the index biochemical oxygen demand (BOD5).

On the basis of this research and the investigations of other authors (1992 - 2002), the paper presents the comparison of the treatment efficiency of domestic wastewater treatment facilities functioning by different natural means. From all the studied three natural wastewater treatment facilities, constructed wetlands are considered as most efficient for domestic wastewater treatment: here the removal of organic pollutants and biogenic compounds is 85-98% and 67-95% respectively. Biological ponds are most suitable for the treatment of domestic wastewater, the discharge and pollution of which is fluctuating within a wide range. However, such ponds do not always ensure the treatment until the allowable rates. Their treatment efficiency according to biochemical oxygen demand (BOD5) is 49.6%. Total nitrogen amount contained in wastewater outflow from the treatment facilities with biological ponds was on the average 22.2 mg lE-1. The biggest problem of those ponds is rather low P removal efficiency (only 29.5%). Therefore wastewater flowing out of the ponds needs additional treatment. The usage of subsoil filtration fields is limited because their arrangement requires light soils. When arranging the distribution pipeline below the freezing zone, in many places the requirement that the pipes must be 1 m above the maximum ground water level is violated. The pipes should be arranged in places where there is no recipient for treated wastewater outflow. The load of subsoil filtration fields should not exceed 300 m**3 haE-1 per month. From the economical point of view, domestic wastewater treatment facilities with flowing ponds require least expenses.

Constructed wetlands (CW) are considered as one of principal wastewater treatment methods containing and soil filters. CW may be of subsurface vertical flow (VF) and horizontal flow (HF). The objective of the studies was to estimate N-removal efficiency differences in CW of vertical and horizontal flow. To evaluate nitrogen (N) removal efficiency, the database of the studies on both construction of CW collected within the period of 1995-2005 was used. Horizontal filter systems characterize for a 1.6 times higher N-removal efficiency than vertical filters. Generally, N-removal efficiency depends on the load of filters. When filter load was 1.5 g mE-2dE-1 according to TN, N-removal efficiency is 39.6 and 24.0% in HF and VF systems respectively. Considering the seasons of the year, no significant difference was observed in N-removal efficiency in HF and VF systems during cold and warm periods of the year. N contained in water after the treatment process in filters is detected in different forms. NH4-N is prevailing in HF, while in VF systems NO3-N is dominant. It was also observed that NH4-N in the effluent during the initial operation of VF after the construction works in the first two months in the warm and in four months during their operation in the cold period of the year was prevailing.