Metals Budget between Wastewater, Treated Water and Soil

In this article we will discuss about the metals budget between wastewater, treated water and soil.

F. Abosamra* and O. Assi

Higher Institute of Applied Sciences and Technology (HIAST),

P.O. Box 31983, Syria, Damascus

Abstract:

This report describes the elimination-concentration procedures for metals in domestic waste water, treated water, sludge and soil. Two types of biological waste treatment were considered: high and medium organic loads.

Three groups of elements were analysed in dissolved and un-dissolved forms:

1. Group I:

Toxic elements, i.e., mercury, cadmium, lead, nickel and chromium.

2. Group II:

Oligo elements, i.e., iron, aluminium, manganese, zinc, copper and cobalt.

3. Group III:

Active biological elements, i.e., potassium, sodium, calcium and magnesium.

Introduction:

Water plays a major role as a vector for organic, microbial and trace metal pollutants. The recycling of treated domestic wastewater has become a crucial ways to conserve water resources and combated water scarcity problems.

Many problems arise from the reuse of wastewater; microbial and trace metals contamination are just two. While disinfection of reused water solves the problem of microbial contamination, the trace metals contamination problem remains unresolved for plants and soil.

Metals reaches a wastewater treatment plant through the sewage from domestic and industrial sources.

These metals can be divided to three categories:

1. Alcalins and earth alcalin metals such as K, Na, Ca and Mg, which are necessary for biological activities,

2. Oligo elements such as Fe, Al, Mn, Zn, and Co, which at high concentrations are considered toxic for living species,

3. Toxic elements such a Hg, Pb, Cr, Ni and Cd.

The metals in category 1 are domestic in origin, while the two others come mainly from industrial sources and as a result of the corrosion of sewerage materials. Metals are present in two forms in sewage streams i.e., those that are soluble in water, and those that are fixed and un-soluble organo-metallic compounds (with protein- metals linkages).

Once the metals reaches the biological treatment plant two types of biochemical processes took place. The first is biochemical elimination of the soluble form through biochemical processes such as linkage to organic material and bio concentration in the living cells. The second is physical elimination of the un-soluble form. The reuse of the treated wastewater for irrigation processes contributes to the elimination of trace metals from the water through the same processes.

The results of this study show that biological wastewater treatment plant processes concentrate and eliminate 50-89 per cent of the metals in the wastewater, depending on the type of metal. Also, the eliminated metals are highly concentrated in the sludge (primary, activated and treated) produced by the treatment processes.

The harm to the environment, especially to plants and soil, results from the reuse of sludge as an organic amendment to the soil. The results show that trace metals contribute to the enrichment of the soil trace metals content. Thus, a scientific and economic approach to the practice of using sludge as an organic source for soil should be considered.

Experimental Work:

The study was conducted in two types of biological wastewater treatment plant:

a. Type I, with an organic load of 1.6 kg BOD/kg vss/d

b. Type II, with an organic load of 0.8 kg BOD/kg vss/d

Samples were taken every 24h from effluent and treated water, instantaneously from activated sludge, and every 15d from primary and digested sludge, throughout the four month duration of the study.

Results:

Effluents:

Table 1 shows the metal contents of effluents in the two forms soluble and un-soluble.

The cumulative contents are 0.3 mg/l for Pb + Mg + Cd + Ni + Cr, and 4.2 mg/l for Cu + Co + Mn + Zn + Fe + Al.

Primary Settling Tank Water:

Table 2 shows the effects of settling on trace metals removal.

Treated Water:

Table 3 gives the metals contents as determined for samples from the two types of depending wastewater treatment plant.

Elimination of Metals during Treatment:

Table 4 shows the elimination of the un-soluble form of metals.

It was difficult to quantify the elimination of metals in the soluble form due to their low concentrations. The results obtained, never the less, are given in Table 5.

Activated Sludge:

Due to the low concentration of metals in treated water, it was difficult to quantify the yield. Thus, the total treatment yield is quantified in Table 6.

Discussion:

The distribution of metals present in effluent depends on the physical and chemical characteristics of the metals. Table 7 shows that Pb, Al and Fe exist in the un-soluble form, and Ni and Mn exist mainly in the soluble form. The distribution, thus, depends on the pH of the precipitation of metal hydroxide in water, which has a pH ranging between 7 and 8. The same phenomena occur in the settled water as occurs in flowing water.

The proportion of the un-soluble form of metals decreases in treated water. Table 8 shows the proportions of the insoluble form in the two organic loads of wastewater treatment plant.

The elimination of soluble Cu, Mn and Fe occurs during settling when the soluble fraction is trapped to maintain the equilibrium between the soluble and un-soluble fraction throughout the mass of water and sludge.

The treatment efficiency for the elimination of metals ranged between 50 and 90 per cent for the metals which have concentrations detectable by Atomic Absorption Spectroscopy.

Different elimination mechanisms could be proposed. The fine particulates contain­ing metals are eliminated after adsorption on the higher floes during the settling process.

The soluble form is eliminated by biosorption. In fact, high biological activity occurs in the vicinity of activated sludge floes. The microorganisms release extracellular organic material which traps or fixes the soluble form of the metals. Activated sludge fauna and fauna bio-accumulate soluble metals in their tissues.

The major conclusion of this paper is that the metals are concentrated in the total amount of the sludge produced by the wastewater treatment plant. The question which is to be asked is, what is the impact of the reuse of sludge for agriculture purposes.

Table 9 shows the average values of the concentration of metals in sludge compared with the concentration of metals found in different soils.

In Table 9, the metals concentration in sludge is shown to be much higher than in normal soil. In other word, the reuse of sludge as an organic amendment for soil causes its enrichment with metals. The determination of the enrichment factors for soil by metals should be considered case by case in order to ensure safe and economic recycling of sludge.

Note: All measurement are on a dry-weight basis.

Many countries establish standards for the metals contents in sludge destined for recycling as a soil amendment as shown in Table 10:

Analytical Procedures:

Figure 1 shows a scheme of the analytical procedures used for the determination of the metal content in different types of samples.