Waste Wash-Water Recycling in Ready Mix Concrete Plants

: The management of waste wash-water (WWW) is one of the most significant environmental problems associated with ready-mix concrete production worldwide. The problems are exacerbated should it be disposed of in an inappropriate manner. This study evaluated the potential of WWW recycling in ready mix concrete plants in Jordan. A representative waste wash-water sample (400 L) was collected from a basin in a ready-mix concrete company. A pilot plant on the lab scale was fabricated and installed. The treatment system consisted of a concrete washout reclaimer, wedgebed slurry settling pond, slow sand filtration unit, and a neutralization unit. Water samples were collected from all stages of the pilot plant and analyzed. The collected waste wash-water samples were utilized for replacement of well water (mixing water) at various ratios. Fourteen concrete mixtures were produced and cast, as well as tested at various curing ages (7, 28, and 90 days). The results show that the raw WWW was not acceptable as mixing water even after dilution as it led to significant reductions in concrete compressive strength and low workability. However, the WWW from the settling pond, the filtered WWW and the filtered-neutralized WWW at dilution ratios up to 75% were shown to be potential alternatives to fresh water for ready-mixed concrete. Therefore, the current guidelines for mixing water quality should be revised to encourage the reuse of the WWW.

heavy metals has been proven as a major threat for human health [4], but concrete has been shown 23 to effectively immobilize any heavy metals within it [5]. The global concrete production is 11 in some countries like the UK and South Africa almost all WWW is recycled [6], The current 28 practices in developing countries is illegal dumping in the city boundaries due to deficiency of 29 government legislative and low care of concrete waste recycling. In many construction sites the 30 construction and demolition waste are mixed together which leads to disallowing of recycling of 31 these parts ([7]; [8]). 32 However, the common practice in Jordan is to send this WWW to landfill or in some cases 33 illegally discharge this near the construction sites. This is a serious threat to the environment and 34 water resources. Ready mix concrete plants are facing an actual challenge due to the water 35 shortage, and high cost of fresh water, and wastewater disposal. Therefore, a novel innovation 36 which inspires new solutions to this challenge will have a direct positive impact on the 37 environment in Jordan and worldwide. In addition, proof of concept of a pilot plant for WWW 38 treatment using efficient, simple and feasible technologies, cost effective and applicable to scale 39 up will lead to produce low cost ready-mix concrete. This work adds to the existing knowledge around WWW use in concrete. This is because the 1 situation in Jordan is different to many other countries that already use WWW in concrete in that 2 the Hellenic standards used for water quality in Jordan are stricter than the EN (EU) or ASTM 3 (USA) standards used elsewhere (see later), and also because the arid conditions in Jordan can 4 result in higher salinity in soils which can be concentrated in WWW and other recycled water [2]. 5 In this study, the optimum goal was to produce zero waste from the ready-mix concrete 6 industry by filtering and treating WWW for reuse as a mixing water in ready mix concrete plants. 7 In addition, the separated solid powder could be collected and recycled in cement clinker or asphalt 8 mixtures. According to United States Environmental Protection Agency [9], the filtered wash 9 water after pretreatment to remove metals and reduce its pH, it can be reused for several 10 applications or it can be delivered to a municipal wastewater network. In addition, cementitious 11 solids can be recycled [ Table 1]. In the UK, according to the Environment Agency [10], it is 12 permissible to treat and reuse concrete wash waters, and cement fines and silt separated from wash 13 waters without an environmental permit as long as the activities do not threaten the environment 14 (water, air, soil, plants or animals) and human health or cause noise or odours that affect the 15 countryside. re-utilize these wastes has attracted wide range of interest, the methods and principles of 19 mechanism of treatments have only been reported in discretely and inconclusive manners.

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Management challenges associated with poor product performance, low re-utilization rate, high 21 cost and strict regulations continue to limit their sustainable utilization.

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The work described in this paper was carried out to characterize WWW from ready-mix Water samples were evaluated for their physical, chemical and biological properties. Concrete 28 mixes using treated water were produced and tested at 7, 28, and 90 days. 29 The expected outcomes of this study are that a novel innovation, which inspires new  during two working days. In order to prevent sedimentation, waste water was stirred for four 4 minutes every 15 minutes. 100 L of raw sample were filtered through slow sand filtration system 5 which consisted of four layers of silica sand, and limestone aggregates (fine, medium and coarse). 6 The aggregate was graded where the fine silica sandstone (the smallest particle size) and the coarse 7 aggregates on the bottom of the basin which act as a drainage system. This simple slow sand filter 8 separated all the suspended solids at the top layer and only permitted the water to pass through the 9 backed layers of aggregates. Finally, the separated sediments were removed and scraped from the 10 top layer. At the laboratory scale, sediments can be removed by manually by simple metal tool. 11 However, on large scale the design allowed a small loader to scrape the sediments from the top 12 layer of sandstone. Figure 1 represents a schematic diagram of recycling process of concrete wash 13 water. 60 L of the filtered WWW were collected from the laboratory prototype filtration system 14 and stored in plastic container. 20 L of the filtered water was neutralized to PH 7 using CO2 gas Three fractions of aggregates (Coarse (9.5-19.0 mm), medium (4.8-9.5 mm), and fine 7 aggregates (<4.8mm)) were used for concrete mixes. The physical properties of the used material 8 were tested according to international standards (ASTM C136-14 [20]) ( Table 2). Moreover, the 9 fineness modulus and the recommended proportion of each aggregate type to be used in concrete 10 mixtures were calculated from sieve analyses data.   A control mix of concrete was performed using the standard mix design which used at a 12 major ready-mix concrete producer in Jordan [ Table 4]. The control mix was prepared by well 13 water as mixing water, as used for normal production at the plant.
14 After the preparation of concrete control mix, well water was replaced by four types of 15 mixing water (raw WWW, settling pond WWW, filtered WWW, and filtered-neutralized WWW) 16 at different percentages (100, 75 and 25%) in separate mixes. The total number of concrete  25% Settling pond + 75% Well water A*: Part of the raw WWW weight is solid content (11.36 wt. %) which was replaced by water to fit the water cement ratio. B*: All the weight of raw WWW considered as water and the suspended solid weight was ignored. All data were statistically analysed using IBM SPSS Statistic Data Editor (Version 19.0).

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Each data point was measured in triplicate. All parameters were fixed and only the type of mixing 20 water was changed. Statistical significance was evaluated by general liner model multivariate --21 Tukey homogeneous subsets multivariate and multiple comparison test at p≤0.05. The chemical and physical properties of the five used water samples are presented in Table   3 6. The results show that the raw WWW was caustic with pH value up to 12.6 (Hydroxide 4 alkalinity). Based on the raw WWW electrical conductivity (Ec) (11854 µS/cm) and the total 5 dissolved salts (TDS) (7097 mg/l), it can be classified as high saline brackish waste water. Klus et 6 al. [13] reported that the raw wastewater from ready mix concrete plant has pH value 12.5 and Ec 7 13390 µS/cm, indicating the results presented here were consistent with previous research. The 8 high pH was mainly attributed to the dissolved alkali hydroxides such as Ca(OH)2, Mg(OH)2, 9 NaOH, and KOH. According to Tsimas and Zervaki [14], the sludge of the concrete washout  The high Ec could be contributed to the dissolved salts and hydroxides as well as the chemical  Moreover, the raw and the filtered WWW are classified as hazardous wastes due to their high 27 pH value (< 9) and not permitted to be disposed in the rain fall drain, valleys, river or on soil 28 according to European, USA, and Jordanian legislation ([14]; JS 202, [29]). 29 The slow sand filtration system using compacted layers of sand stone and limestone   The water quality results in Table 6 show that the Filtered-Neutralized WWW using CO2 gas 1 meets EN, ASTM and Hellenic Standard limits for mixing water. Moreover, it meets all the 2 maximum limits of the Jordanian Standard Specification for industrial reclaimed wastewater that 3 effluent water is discharged into valleys and streams [29] except the maximum limit of Hg. 4 Therefore, this reclaimed WWW should be recycled as mixing water and should not be discharged 5 into the environment.    Figure 2 shows the slump of each fresh concrete sample. The slump value of the concrete 2 mixtures using filtered WWW (F100, F75W25, and F25W75) or filtered-neutralized WWW 3 (FN100, FN75W25, and FN25W75) at various concentration (100, 75, and 25% of total weight of 4 mixing water) show slight reduction in comparison with the control mixture (W100). In addition, 5 use of WWW from the settling pond (S100, S75W25, and S25W75) showed no significant 6 differences in comparison with the control mixture. Moreover, it is clear that the using raw WWW   The statistical significance differences in compressive strength at 7, 28 and 90 days were  The results of statistical significance differences of concrete compressive strength revealed 9 that in comparison with control (W100) which was prepared by well water, there were no 10 significant differences with all mixtures except S100, S75W25, S25W75, and B-R100. Moreover, 11 S100, S75W25, S25W75 show reduction in relative strength index by -13.1, -18.9, and -10.8%, 12 respectively. In addition, B-R100 show increase in relative strength index by +13.9%.  The results of statistical significance differences of compressive strength at 28 days revealed 7 that there were no significant differences between the compressive strength of the control 8 specimens (W100) and all mixtures except Mix (A-R100) and Mix (R75W25). The Mix (A-R100) 9 and Mix (R75W25) showed the same reduction in relative strength index by -10.66 % (Table 8). 10 Mixtures A-R100, B-R100, R75W25, R25W75 were made with raw WWW. The only 11 difference between Mix (A-R100) and Mix (B-R100) is that in Mix (A-R100) part of the raw 12 WWW weight was solid content (11.4 wt. %) which was replaced by water to fit the water cement 13 ratio while in B-R100 all the mass of Raw WWW was considered as water and the suspended solid 14 mass was ignored. In comparison to A-R100, B-R100 had less free water and more fine powder.

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Although, using raw WWW in B-R100 led to a significant increase in the concrete compressive 16 strength, it was less workable and had a slump of zero. In comparison with A-R100 at curing time 17 7, 28, and 90 days, dilution of the raw WWW with well water at ratios of 3:1 (R75W25) and 1:3 18 (R25W75) led to no significant differences in compressive strength. In addition, the slump 19 increased with an increase in the dilution ratio, but the workability remained low in comparison 20 with the control mixture. One possible explanation is that the mixing water sample used for B-21 R100 mixture contained more suspension solids (fine powder) and less free water than that 22 required to meet the water cement ratio. Subsequently, B-R100 had zero slump and very low Statistical significance of compressive strength at 90 days was evaluated by general liner 8 model multivariate -Tukey homogeneous subsets ( Table 9). The results showed that there were 9 significant differences in compressive strength between the control specimen (W100) and the 10 following specimens, FN100, A-R100, B-R100 and R75W25, with concrete relative strength 11 index of -8.7, -12.7, +7.7, and -10.7 %, respectively. 12 The statistical significance differences of concrete compressive strength at 90 days curing 13 time was evaluated by Tukey multiple comparison test at p≤0.05 (Table 10). It revealed that there 14 were no significant differences in compressive strength between control specimens (W100) and 15 the following specimens: F100, F75W25, F25W75, FN75W25, FN25W75, R25W75, S100, 16 S75W25, and S25W75. In addition, there were significant differences with specimens FN100, A-17 R100, B-R100, R75W25. The significant mean difference ranged from -3. 8  In comparison with control specimens, the statistical results of compressive strength of 3 mixtures which were prepared with WWW from the settling pond at various dilution factor showed 4 that there was significant reduction in compressive strength: with reductions for S100, S75W25 5 and S25W100 of -13.1, -18.9 and -10.8% at curing time 7 days. However, there were no significant is suitable for concrete production if concrete made with it have compressive strength reduction 8 less than 15% in the mean compressive strength of concrete specimens prepared with drinkable 9 water or distilled water. Therefore, the raw WWW and its dilution at ratio 3:1 are not accepted as 10 mixing water due to high reduction of compressive strength and the low workability of concrete 11 made by it. It clear that dilution of the raw WWW is also not a solution. wastewater in concrete led to a significant strength reduction of 19.6% and 10%, respectively. In 18 addition, reusing raw grey water and treated grey water in concrete led to a significant reduction 19 in compressive strength up to 13.9% and 2.4% at curing times up to 200 days, respectively. They 20 concluded that treated wastewater and grey water can be suitable for concrete production.   This study has shown that raw Waste Wash Water (WWW as it comes from a mixer vehicle) 3 does not meet the current standard maximum concentrations limits for concrete mixing water 4 according to EN, ASTM or the Hellenic standard used in Jordan. Test results have shown that this 5 raw WWW also leads to significant compressive strength and slump value reductions as well as 6 poor workability, even after dilution. Moreover, this raw WWW meets the classification for a 7 hazardous waste in Jordan. When the WWW had larger particles removed in a settling pond, it still 8 did not meet the water quality standards. However, when it was used to replace 75% of the mix 9 water it did not result in statistically significant strength reductions at the 95% confidence level, 10 and there were no major implications for workability. Consequently, this indicates that the EN and 11 ASTM standards for mix water may have a degree of conservatism built into them since water 12 outside their boundaries appears to be at least partially suitable as mix water.

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When the water was passed through the settling pond was filtered, the quality did improve 14 to the point where it met the EN and ASTM standards for mix water, but still did not meet the 15 Hellenic standard, mainly because the pH was still too high. As with the water in the settling pond, 16 there was no significant strength reduction or workability concerns when it was used as mix water.

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In order to meet the Hellenic standard, as well as the Jordanian Standard Specification for