Effects of Three Kinds of Sewage Sludge on Physiological Characteristics of Flower Plants

Yanyan HU Xiang CHEN Fuyin XU Bing BAO

Abstract Effects of dried sludge， digested sludge and composted sludge （obtained from the raw sludge of a large sewage treatment plant in Chongqing City） on the physiological characteristics of Salvia splendens， Tagetes patula and Petunia hybrida were studied. The results showed that the application of sludge could significantly promote the growth of three kinds of flowers and improve their quality. The effects of sludge application rate on the biological characteristics of three kinds of flowers were significant. The optimal application rate of the sludge for the growth of three kinds of flowers was 50% （mass percentage） for S. splendens and 37.5% for P. hybrida and T. patula. The effect of sludge type on the biological characteristics was not significant. However， among the three types of sludge， composted sludge had the highest organic matter content and cation exchange capacity and the lowest salt content， so the improvement of garden soil by composted sludge was relatively significant and safe.

Key words Sewage sludge; Heavy metals; Flower; Physiological characteristics

Sewage sludge， the sediment produced in the process of urban sewage purification[1]， and its output is huge and increases rapidly. It has become a new source of pollution and posed a great threat to urban environment， so it is imminent to solve the problem of sludge disposal[2-3]. The sludge from municipal sewage plants is rich in available nitrogen， phosphorus， trace elements and organic matter but contains heavy metals， pathogens， toxic and harmful organic pollutants， etc.， which restricts the resource utilization of sludge[4-5]. Utilization of sewage sludge in gardens has the advantages of low cost and no entry into the human food chain. Its application technology has been widely studied by scholars at home and abroad， and sewage sludge has been successfully applied in gardens in cities such as Beijing， Shanghai and Shenzhen[6-8]. The treated sludge product can be used as both a cultivation medium and an organic fertilizer. In this paper， effects of three kinds of common sludge in Chongqing （dried sludge， digested sludge and composted sludge） on the growth of Salvia splendens， Tagetes patula and Petunia hybrida were studied to provide theoretical and practical support for the application of sewage sludge in production of garden flowers.

Materials and Methods

The tested sludge and soil

The tested sludge was the raw sludge of a large sewage treatment plant in Chongqing City. The dried sludge was obtained after the raw sludge was dehydrated and airdried. The digested sludge was obtained after the raw sludge was digested by the sewage treatment plant. The composted sludge was obtained after the raw sludge  naturally rotted for 3 months. The tested soil was ordinary potting soil， which is composed of nursery native soil， decomposing bacteria package and decomposing farmyard manure according to the volume ratio of 80≥15≥5. The physical and chemical properties and heavy metal content of the tested sludge and soil are shown in Tables 1 and 2.

Experimental design

The test was carried out in the shelter of flower base of Chongqing Ruisheng Company in Baishiyi， Jiulongpo District， Chongqing City. The test adopted a completely randomized block test design consisting of two factors and three levels. There were three treatments of sludge types， including dried sludge （T1）， digested sludge （T2） and composted sludge （T3）. There were three treatments of sludge application rate， including 25% （D1）， 37.5% （D2） and 50% （D3）. Three blocks were set up in the experiment， and ordinary potting soil （CK） was as a control group in each block. There were 30 repetitions in each treatment. The sludge and the common potting soil were mixed in proportion and placed for two weeks， and the  seedlings of flowers were planted in pots.

Determination indicators and methods

The physical and chemical indicators of the soil and sludge were determined as follows： bulk density was measured by cutting ring method; pH was determined by potentiometry; total amount of watersoluble salt （electrical conductivity of soil solution） was determined by conductance method; organic matter was measured by potassium dichromate volumetric method; available nitrogen was determined by alkali diffusion method; available phosphorus was measured by molybdenum blue colorimetry; available potassium was determined by flame photometer; cation exchange capacity （CEC） was measured by ammonium acetate method; heavy metals were determined by atomic absorption spectrophotometry. The physiological characteristics of flower plants were measured as follows： in the fully flowering period， 10 plants were randomly selected from each treatment to measure the number of flowers， flower diameter， ear length， crown width， plant height and aboveground biomass.

Results and Analysis

Effects on the physiological characteristics of S. splendens

After the three kinds of sludge were applied to S. splendens， the crown width increased， but the plant height decreased， and the ratio of plant height to crown width was more coordinated. The plant height was mainly affected by the types of sludge， and there was not significant difference between the effects of sludge application rate on the plant height. For different types of sludge， the difference between dried sludge and digested sludge or digested sludge and composted sludge was not significant， but the difference between dried sludge and composted sludge was significant. The crown width was mainly affected by sludge application rate， and there was not significant difference between the effects of different types of sludge on the crown width. With the increase of sludge application rate， the crown width increased continuously. The difference between 25.0% treatment and the control group was not significant. When sludge application rate was 37.5%， the crown width increased significantly compared with the control group. As sludge application rate increased to 50%， the crown width of plants in the treatments with dried sludge and digested sludge decreased， while the crown of in the treatment with composted sludge continued to increase， indicating that when the application rate of dried sludge and digested sludge was 50%， the growth of the aboveground part was inhibited. Crown width and plant height are the two major factors that contribute to aboveground biomass. The application of the sludge could promote the growth of aboveground part of S. splendens and increase the aboveground biomass significantly. The aboveground biomass was the highest in the treatment T1D37.5， which was 150% higher than the control group. It was the lowest in the treatments T1D50 and T2D50， which was 87% higher than the control group. After the application of the sludge， the number of spikes of S. splendens increased significantly， and the length of the spikes reduced significantly. However， the flowering of S. splendens was mainly affected by sludge application rate， while the effect of sludge type was not significant.

Effects on the physiological characteristics of T. patula

Compared with the control group， the applocation of the sludge could promote the growth of aboveground part of T. patula significantly， that is， the aboveground biomass， crown width and plant height increased significantly. The application of the sludge promoted the flowering of T. patula and increased the number of flowers and flower diameter， thereby improving the ornamental quality of T. patula. Sludge type had no significant effect on the quality of T. patula. For sludge application rate， there was no significant difference between the treatments 25% and 37.5% in respect of the number of flowers， flower diameter， crown width and plant height， while they reduced significantly in 50% treatment compared with the first two. There was a significant difference between the treatments 50% and 37.5% in terms of the aboveground biomass， but the difference between the treatments 37.5% and 25%. This shows that the quality of T. patula plants （including aboveground biomass， the ratio of plant height to crown width， etc.） and flowering quality （number of flowers and flower diameter） were improved with the increase of sludge application rate from 0% to 37.5%. However， when sludge application rate was 50%， the plants were obviously poisoned， and the quality of the potted flowers significantly decreased.

Effects on the physiological characteristics of P. hybrida

the applocation of the sludge could promote the growth of aboveground part of P. hybrida significantly， and the aboveground biomass increased significantly， but it has no significant effect on the crown width， plant height and flower diameter. The difference between the effects of sludge type on the quality indicators of P. hybrida was not significant， but sludge application rate had significant effect on the growth of P. hybrida. In comparison with the control group， the aboveground biomass and the number of flowers increased significantly in the treatments with the sludge. With the increase of sludge application rate from 0% to 37.5%， the number of flowers rose. When sludge application rate increased to 50%， the number of flowers dropped significantly. When sludge application rate was 25%， the increase of the biomass in 25% treatment was the largest. As sludge application rate increased to 37.5% and 50%， it declined gradually. Compared with the control group， there was no significant difference between various treatments with the sludge in terms of flower diameter. When sludge application rate was 25% and 37.5%， the flower diameter was large. As it was 50%， the flower diameter reduced obviously. It reveals that when sludge application rate increased to 50%， the toxic effect of the sludge on P. hybrida was obvious.

Conclusions and Discussion

Sludge contains a large amount of abundant available nitrogen， phosphorus， trace elements and organic matter， so it can improve physical and chemical properties and biological properties of soil， and can be used as fertilizer and soil conditioner[4，9]. In this study， the results showed that compared with common potting soil， the application of sludge could significantly promote the growth of three kinds of flowers and improve their quality. This is because the organic matter， available nitrogen， available phosphorus and cation exchange capacity of three kinds of sludge were higher than those of ordinary potting soil， and the effect on soil improvement was obvious， thus promoting plant growth.

Heavy metals in sludge are the main obstacles affecting sludge land use[10]. Although the zinc， mercury and cadmium content of the tested sludge exceeded the grade 3 standard of soil environmental quality， they did not exceed the national standard of sludge for agricultural use. Moreover， garden plants can accept higher concentrations of heavy metals and are less prone to toxic symptoms[11]. In this study， the results indicated that the effects of sludge application rate on the biological characteristics of three kinds of flowers were significant. With the increase of sludge application rate， the changes in the biological characteristics of various kinds of flowers were different. For the ornamental quality， the effect of applying sludge on the physiological characteristics is as follows： T. patula>S. splendens>P. hybrida. According to the growth of the aboveground part and flowering quality， the optimal application rate of the sludge was 50% for S. splendens and 37.5% for P. hybrida and T. patula， and the growth of the flowers would be inhibited when it exceeded the highest value.

Composting treatment can reduce the activity of heavy metals and maintain nutrients to a certain extent， so that the content of heavy metals that can be absorbed and utilized by plants is reduced， thereby achieving the purpose of land use[12]. In this study， the results indicated that the effect of sludge type on the biological characteristics of the three flowers was not significant. Among the three kinds of sludge， copper， zinc and mercury content in the treatments with composted sludge were the lowest. This may be because the composting process of composted sludge in this study was relatively simple， so that it was decomposed to a certain degree. However， among the three types of sludge， composted sludge had the highest organic matter content and cation exchange capacity， and the fertilizer efficiency was more stable. The improvement of garden soil by composted sludge was more significant， and the salt content of composted sludge was lower than that of the other sludge， so it was relatively safe. In practical applications， straw， wood chips， lime， etc. should be added to sludge to reduce the effectiveness of heavy metals and promote the growth of flowers.

References

[1]SHI XL， CHEN SW. Utilization and treatment of municipal sewage sludge[J]. Environmental Protection，2001（3）： 45-46. （in Chinese）.

[2]LI YQ， CHEN L， ZHAO JF， et al. Rapid and highefficiency composting of sewage sludge[J]. Journal of AgroEnvironment Science，2005，24（2）：380-383. （in Chinese）.

[3]YUE XH. Experimental study on the application of municipal sludge in garden plants[J]. Ningxia Journal of Agriculture and Forestry Science and Technology， 2006（3）：15，26. （in Chinese）.

[4]ZHOU LX， HU AT， GE NF， et al. Study on utilization of municipal sewage sludge in farmland and forest land[J]. Acta Ecologica Sinica，1999，19（2）：185-193. （in Chinese）.

[5]QIAO XL， LUO YM， WU SC. Land use of sludge and its environmental impact[J]. Soils，2000（2）：79-85. （in Chinese）.

[6]FANG HL， CHEN H， ZHANG Q， et al. Discussion on the application effect of urban domestic sludge in  urban green space of Shanghai[M]♂ZHANG C. Research progress in sludge treatment and disposal technology. Shanghai： Chemical Industry Press， 2005. （in Chinese）.

[7]LIN LW， ZHONG JH， ZHANG GL， et al. The application of compost of city sewage sludge to environmental greening[J]. Ecology and Environmnet，2006， 15（5）：974-978. （in Chinese）.

[8]LI GB， YIN CQ， SHAN BQ. Land use and prospect of sewage sludge in forestland and green areas[J]. Journal of Beijing Forestry University，2001， 23（4）：71-74. （in Chinese）.

[9]SUN Y， CHEN L， ZHAO JF. Study progress on sludge treatment and disposal and its utilization on agriculture[J]. Environmental Protection Science，2002，28（114）：28-30，37. （in Chinese）.

[10]PANG JH. Effect of sewage sludge on regional ecoenvironment[J]. Tropical and Subtropical Soil Science，1994，3（1）：41-47. （in Chinese）.

[11]WANG X， ZHOU QX. Effects of land utilization of sewage sludge compost on trees growth and soil environment[J]. 2005， 24（1）：174-177. （in Chinese）.

[12]LIANG L， ZHAO XL. Changes of nutrients and heavy metals contents in sludge after composting[J]. Environmental Science and Management，2006，31（1）：63-65. （in Chinese）.