2 Trench system using composite soil for advanced treatment of municipal wastewater
Masataka Sugahara

Department of Human Environment,
Osaka Sangyo University

1. System structure
    There are two kinds of trench soil percolation equipments being utilized in this experiment. One is called Trench-1 and the other Trench-2. Each equipment differs in its composition element and capacity (Ref. Fig.1, Table 1).In Trench-1, there is only one trench and the soil layers consist of the composite soil, air layer, composite soil again, filter layer, and storage layer from top to bottom. On the other hand, Trench-2 has two trenches, while the soil layers are composed of the composite soil, filter layer, and air layer sequentially from the top. The composite soil has different particle size and layer thickness in the two trenches as will be mentioned later.In addition, Trench-1 is 5m in length, a little longer than a regular trench, which is 4m only.

Fig.1 cross-section view of trench
Fig.1 cross-section view of trench

Table-1 Composition,property,and thickness of compo

       In this trench system, some improvements are added to the conventional trench equipment. The most important of which is the application of the comosite soil.
    The composition, effective diameter, and unit weight used for the verification test of this facility are shown in Table 2. The soil composition includes inorganic substances, such as decomposed granite soil, river sand, and pumice stone; organic substances, such as mulch, bark compost, and poultry manure; and activated carbon. Inorganic substances comprise about 83% of the soil composition. In order to decide for the composition of composite soil, improved ventilation state and longer duration of the processing function were considered. By placing an air layer at the lower part below a trench, the result is an increased air permeability in a soil layer, thus, an aerobic state can be maintained. With this procedure, progression of clogging can be controlled and functional recovery time can be shortened.
Table-2 Target quality classified by use and water quality acquired with the experiment facilities

2. System performance
    The operation of the equipments adopts a daily pattern of an 8-hour pause after a 16-hour operation. Inflow water goes into a trench intermittently. The pump operation is set up according to each inflow once every 60 minutes. Moreover, the operation pattern during the whole term is a two-month operation and one-month pause. For the whole term, the operation functions for about eight months from summer to the spring of the following year.
    In this experiment, the inflow water used is withdrawn from the distribution tank at the sewage-treatment plant and treated in the septic tank. That is, it is the water from treated municipal wastewater. A result is reported as follows. The throughput is expressed in the treated quantity () per 1m2 a day. In Trench-1, the treated water remained around 80/m2 a day. Although, it temporarily dropped to 70/ m2 a day, the stable supply of treated water was maintained due to the recovery effect during the pause phase. In the contrary, the treated water in Trench-2 was 80/m2 a day in the beginning. The amount rose immediately and reached 110/ m2 a day temporarily. Afterwards, the treated water was maintained over 100/m2 a day until the period of dormancy. For the time being, the high throughput before the pause period maintained a similar quantity even after the pause phase. However, it dropped rapidly after that and was reduced down to 50/m2 a day. The 2nd break started, as exactly planned at this time. This second pause resulted in the recovery of 80/m2 a day throughput as the initial treatment. Afterwards, the throughput was stabilized. In addition, it was confirmed that the rapid drop of treated water was because it mixed with the sludge at the bottom of the flow regulating tank and an inadequate measurement at the time. In spite of such an unexpected situation, recovery of water quantity did well eventually.
    According to the average throughput during the investigation period, 77.1/m2 a day was treated in Trench-1, while 80.2/m2 a day in Trench-2. There was no big difference between the two equipments. The slightly thinner layer of composite soil and larger grain size in Trench-2 as compared to Trench-1 resulted in the increase of treated water. However, the effect was small and temporary.
    In order to confirm the purification ability of the trench system in terms of suspended solids (SS) and biological oxygen demand (BOD), which are basic factors of the purification ability, SS is sharply changed by the inflow water. However, the rate of removal of the trench system is high and below 5mg/ SS is basically maintained in the treated water. The value of BOD is altered by the inflow water, as well. Similarly, the rate of removal is high.. BOD is 0.5mg/ in Trench-1, while below 1.4mg/ in Trench-2. The system’s highly purification ability is observed.
    When reuse of water is considered, the factors for water quality which should be examined in addition to BOD and SS, are COD, coliform bacteria count, turbidity, and chromaticity as listed in Table 2. Subsequently, a review of the purification characteristics for water quality using Trench-2 is described.
    As regards COD, the high-quality treated water nearly remained below approximately 10mg/. Even if the quality of inflow water (pre-treated water) changes sharply, it has enough performance to respond to it. A stable result of coliform bacteria count was not obtained in the early stages of operation, subsequently below 10 coliforms /m was basically maintained during the stable period. Turbidity dropped to 3 degrees or less and remained stable. Chromaticity ranged between 6 and 24 degrees (average: 13.3 degrees).
    Finally, T-N, T-P, and transparency of treated water, which are not included in the factors for water quality on the guideline about the reuse of water, are discussed. In T-N, seasonal changes of removal capacity were seen: below 3mg/ in winter, while 6 - 7mg/ in summer. The rate of removal was low. In T-P, the result showed close to below 1mg/. Approximately between 80 and 90% was maintained as the rate of removal. Transparency was between 19 and 100cm. A higher value in wintertime than in summertime was obtained.
    The above results indicate that the quality of treated water is comparatively stabilized according to the change of inflow water concentration. The result is considered satisfactory.However, it is clear that water quality needs further improvement because of the result in the coliform bacteria count, chromaticity, etc., depending on the application. In addition, satisfying data concerning the amount of treated water is not obtained. In order to solve these problems, new developments on substances composed of soil and its component ratio, filling system and ventilation system are expected.

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